Experience and even then the experts sometimes get taken.
Realize several things - most fake 'wear' does not look right
most freshened up guns look freshened up
actual patina is almost impossible to fake
buying from known parties is best - however, are we talking $1000 guns or $15,000 guns? The less expensive guns are unlikely to be faked, why, because they already cost $800-$1500 to buy, what profit margin is there in doing so?
When a new stamp (and most on ebay look fine to poor) - the stamp is applied over the blue, which is not how it would have been done. So, it has to be reblued, rebluing process involves what?
So, to fake something is possible and there are several folks who do it as a living, but most of the time you can tell.
My bigest concern is the vast majority of folks who want a 98-99% gun. They exist, but come on folks, no wear on a 80-100 year old gun? Yes, I have seen very nice ones, but like I said, too many folks want a pristine gun and they get all excited over 'pretty gun'. Heck of a lot of VERY nice Test Lugers out there IMHO
Ed