There is really no crash course for this, I've been here over a year and I'm still learning.
If you need to quickly dicern is a sub is fake (for purchase) then as Pugwash said the best way is to open her up.
In general though the list is large, Mag on the date window, font on the bezel, number of clicks of the bezel, pearl, case shape, font on dial, does the M's line up, crown guards, swiss made over 5 ticks, correct bracelet and the list just goes on and on.
I suggest that you start off spending some time in the Auction Scam section, you will learn a lot in there.
Ken