Some are incredibly obviously fakes. Super glossy paper front and back, lousy quality.
Others are really good. I have had occasion to have real ones and great fakes side by side, and only then can you see the clear differences: the cropping is sometimes a tiny bit different, the printing quality is slightly fuzzier on the fake, and the backs of the fakes have a slight glossiness that is different from the backs of the real ones.
I can't guarantee you can always tell a fake from a real one, but follow these two tips and you will eliminate 99% of the worry:
1) Never ever buy an insert from Rochester New York or the two or three other main eBay dealers known to sell fakes (see any thread on this subject)
2) Only buy from any dealer who promises an unlimited guarantee that you can return it at any time in the future if it turns out to be a fake (I offer this, and I imagine several others would as well if you ask them). Of course, such a guarantee is only as good as the seller, so don't ask for this from a zero eBay feedback seller.
It is not as worrisome as you make it sound, but know that collectors have been bilked out of many hundreds of thousands of dollars on eBay alone, and no one has ever been punished in any way (except they give refunds to keep their 100% positive ratings).
Bruce