The only real way, if it's an option for you, is to take your terp/handler and go down town or better yet, get out of the city and have them take you to a real arms dealer (gun store) in a smaller town. Additionally, all of the foreigners are paying way too much (as in relative fortunes) at the bazaars, but since they will, that's the market price.
After putting significant effort (with stateside, AF and PK help) into identifying the fakes from "real", I've come to the conclusion that unless you are truly an expert/collector in the type of firearm you are looking for, real from fake cannot be determined. Even if you find an information source for all of the proofmarks found on a weapon, the smarter manufacturers know this as well and make sure their copies have all of the correct marks. Obviously I'm talking about good fakes...the Enfields with backwards "N"s are obvious fakes.
That, and the 'ghans can make a decent copy of just about anything.
I've dug around in a lot of weapons storage "yards" (with everything from sabres to FROG missiles) and even in the weapons siezed from the warring factions when the Northern Alliance and AMF were dissolved you often find fakes. In other words, guys were carrying whatever they could bet their hands on, "real" or otherwise, and they all got scooped up. I once went in a yard with a British LTC who was looking for some Enfields to make a plaque for his general and after going through a couple of conex boxes of them he gave up and just picked the two best looking ones.
Be certain of what you will be allowed to ship; when the 82nd rolled in a year or so ago they significanlty curtailed what can/not be shipped...no more Enfields, Martinis, etc. You are basically left with the flint locks/muzzleloaders.
My advice would be to haggle down to a good price on something you like the looks of and hang it on your wall as a souvenir...after all, "real" or otherwise it came from Afghanistan.