Warning: This is a long & big post
I've recently had the chance to "slowly" compare a fake U-Boat of my original U-Boat and I wrote down my observations so that if anyone is in the U-Boat market and wants to know what to look for before they buy - to ensure they buy the real thing - this should help (and in some regards this may help for non-U-Boat watches as well). This will of course be slightly biased towards the watch I have (the Flightdeck CAB 50 WH - WH stands for White, 50 stands for the size in mm). The fake I compared it to is considered a "very good fake", meaning it actually costs a bit. The original is in the $3600 range whereas this fake is about $400.
I'll use pictures to show differences wherever possible. When I say Catalog I mean either the U-Boat catalog or you can usually get very clear pictures from the official website which is located here.
Easy to spot differences
1. (CAB 50 WH specific). In the case of the white watch, it comes with a white crocodile "racing" strap that has 3 different-sized holes in it (on each strap part) and is hand sown. This is the absolute easiest way to spot a fake, as to my knowledge and my Googling, no fakes show up with that strap (the strap by itself retails for about 250 Euro). Perhaps it's just hard to fake it as "white crocodile" doesn't come cheap. The fake U-Boats I've seen online are all equipped with a black strap. For this watch, this is of course wrong. The white strap is thicker at the top and thins out slightly towards the clasp which has U-BOAT engraved on it. (It is possible to order the strap from U-Boat separately but so far no fake watch sellers seem to go to that length). Even the fake black strap is of poor quality, but this may be hard to spot from photos.
2. The spacing between the "9" and the "U-BOAT" text is different on the real vs. the fake watch (it should be very close to the 9)
3. The fake one has a depressed second area (like the minute/hour circles) whereas the real one does not, and only has it on the minute/hour circles (this differs from fake to fake and from U-Boat to U-Boat, check the catalog to confirm.).
4. Movement. I've yet to see a fake that has a proper Swiss Valjoux 7750 in it. The Asian Valjoux 7750 is different in some regards and to a trained eye it's very easy to see the differences, but let's go over them anyway;
1. Screws. Look at the U-Boat catalog/website to see what color screws your watch is supposed to have. A lot of Asian Valjouxs use blue screws throughout. In my original they are just silver, so that's an instant giveaway (but some boats may very well use blue screws).
2. Sometimes they do and sometimes they don't, but various parts of the metal inside the movement is sometimes brushed or patterned on the Asian 7750, this is wrong in the U-Boat Valjoux case.
3. The Asian 7750 has no "balance bridge Etachron system", or "hammer" as I like to call it (not sure what the proper name is of that exact part, but it's the little pointy thing that points to a small adjustment scale just next to the golden balance wheel that is continuously moving).
4. This might be hard to spot unless you're holding the fake in your hand. The engraved rotor should only wind the watch (clicking noise) in one direction and should be hard to keep in 1 position when titled over, it should somehow prefer certain positions (this may be true for all Swiss Valjoux 7750's, I'm not sure). The Asian one gladly puts the rotor wherever you tilt the watch and as far as I know, winds in both directions without any clicking noise. So if it's smooth, it's a fake or at the very least the wrong movement.
5. No visible ETA engraving or Jewels engraving, perhaps the easiest one to see with the naked eye. As far as I know, all at-ETA-engraved 7750's have their engravings in the same place (I read somewhere you can buy them without engravings).
6. This only applies to non-lume U-Boats and is blatantly obvious. If the watch you see has lume and the original doesn't, fake (but kind of nice, I wish mine had lume...).
Slightly harder to spot differences
1. The main hands. On the real boat, they are glossy on the colored part (in this case the black part), and on the fake they are matte. The numbers on the dial are also glossy and slightly elevated, on the fake they are as well but the fakes tend to have a poorly applied edge and it looks smudged when one looks closely. So unless the hands shine in the flash of the camera or visibly in the picture or when you hold it, or the numbers look smudged, it's a fake.
2. The dial. On the black-dial U-Boats this one might be hard or impossible to spot. On the white one, the "white" should really pop. It should be as white as a white paper. The fake one (and this may be the glass causing light-discoloration as well), is slightly off-white. Unless it's compared to "bright white color" it might be hard to spot, but if it doesn't look super-white to you, it's probably a fake.
3. The case. The black PVD U-Boat case is very black. Pitch black including various text inscribed on the case. And this is important as many fakes get this wrong. The inside of the engraved text should be black too. If you see any other color (probably silver or poorly applied PVD), it's a fake. Also, if the case looks dark gray, or just not black enough, it's a fake.
4. The tip of the second hand (Chrono). The very tip of the tiny second hand should stick out rather oddly, perhaps a millimeter or two extra, almost like a pointy noise. This seems to be something the fakes have a hard time with and the tip is more like a normal triangle tip at the top. It may be hard to see in pictures unless they are really clear and zoomed in. Again, this hand along with the other chrono hands should be glossy and not matte on the colored parts of the hands.
5. The "U-BOAT", "ITALO FONTANA" and "MADE IN ITALY" dial text. This text should not be bold, it should also be slightly gray (on the white-back one, not sure on the other colors). The coloring the fakes seem to get right, but the font is nearly always wrong even though it is very close. Take a close look at it on an original (picture or real watch) and compare it to what you're looking at. If you see any difference, fake.
Hard to spot differences
1. Depending on the U-Boat version you have this may or may not apply (check the U-Boat catalog pictures). On my watch, the rotor that has the U-Boat engraving is colored. This may need a closer inspection, but for example on the CAB 50 WH, the "U-BOAT" and "ETA 7750" text on the rotor is in gold, not silver. Fakes never get this right from what I've seen. The rotor is different for each boat, so check it closely in the catalog so you know what to look for.
Holding it in-your-hand differences
1. The big crown or "cover crown". Unscrew it to reach the real adjustment crown. When you screw the big crown back in and you reach "the end", it should tighten as if it was a seal, so it tightens slowly and smoothly. On the fake it's more like it really hits the end and just suddenly doesn't go any further. This is wrong.
2. Touch it. Touch the case, and especially touch the engraving of "U-BOAT" on the side of the case, let your finger run over it. Feel the grooves. If there's any sharp edge or your finger gets stuck on any engraving whatsoever, it's a fake. The real U-Boat is very smooth.
3. The fake will feel cheap. I don't know how to describe this better, but if you get the feeling it doesn't "feel right" somehow, go with that feeling and put your wallet back in your pocket.
4. The glass. Modern U-boats use crystal sapphire glass (although I know older versions did not). If you have the watch in hand, drop a droplet of water on it. If it's CS glass the droplet will stay in one spot and it's hard to break it up (you can pretty much push it around). If it's normal glass the water will just spread out. I find this to be the easiest CS-test. Most "advanced" fakes seem to use CS glass however.
1. As the fake case is not the proper material it has a tendency to discolor (to a grayish color) when it's exposed to air/elements. This can be cleaned off rather easily, but it happens very quickly again. On the real case you can get the same effect, usually where the strap touches the case (if you have a thick strap), but that should be pretty much the only place you see it as the real case does not discolor. I noticed on the fake it sometimes "blemished" after just an hour of outdoors use.
1. Even if you buy used, it should come with a nice U-Boat marked leather box and papers, and a credit card sized plastic card with a serial number on it and other numbers, a proper manual (generic for any u-boat as it describes each different type and how to set time & wind it etc). Mine came wrapped in U-Boat stamped paper too. If your watch arrives without any of these "obvious" things, send it back and get your money back. I wouldn't buy a used U-Boat if it was missing the original leather box and especially the serial-number card (although this seems like a very fake-able item). Neither should you. From what I know, fakes usually arrive watch-only. It seems U-Boat and Italo are finally understanding that engraving a serial number on the watch might be a good idea, and from what I've read, future watches will have serial numbers on them (not sure if this applies to existing versions or just future versions).
That's about it. I hope this post helps anyone in the U-Boat market. There's so many fakes out there and they're only getting better, so be careful. And please keep the U-Boat bashing to a different thread