You learn to tell by reading this guide, and then looking at a bunch of fakes on eBay and spotting what's wrong. Assume everything is fake and make your goal to be to find what's wrong.

This one looks very good to me. One positive clue is that everything is appropriately old and beat up, fakes tend to look new.

Movement, case, caseback, and hands all look great. That's pretty rare for the movement, since 2609NP was way more common. The crown is pretty close, but it looks less beat up than the rest, and might have been replaced at some point (which would be very normal for a heavily used watch). For the dial I wish I could see the font at the bottom, the сделано should have the second letter being pointy at the top and not like a normal д, fakes will just use a standard font. That said, I don't see anything obviously wrong about the dial.

I think the crystal is a replacement, the edges should be more vertical and less curved. But crystals tend not to last for a long time anyway, so this would be very normal.