Lots of people ask about dating watches so I thought it might be a good idea to share our collective information.
I'll start the ball rolling with dating Omegas. Essentially all Omegas are dated by their movement number, obviously you'll need access the movement to find this. Once that's done here's a list of movement numbers with corresponding years, I hope this is legible.
Although this is a good system and extremely useful it's not infallible or 100% accurate. Lots of Omegas are put togethers or frankenwatches especially the more iconic models. For example a vintage SM300 may well have a cal.565 from a much less valuable model like a Geneve. If your movement has a R next to the number it means it's a service replacement movement (thanks to JonW for that snippet of info).
Hope this helps and if anyone want to share dating information from other manufacturers please add to this post.
I've got a scan of a German Tissot dating sheet somewhere, if I can find it I'll translate, type it out and post later.