If you want to check/test the Auto Light Sensor, you need to enter the test mode first. All you need for that is to press simultaneously "MODE", "ADJUST" and "LIGHT" buttons. You will end up with seeing something like this:
After that you just tilt the watch till you see the digits 88 88 appeared on screen. It will look like this:
That's it actually. You can play with the watch in this mode, to know exactly when the Auto Light Sensor is on or off.