There are plenty of legit affiliate programs that pay...My favorite programs (actually networks) in order are AzoogleAds, NeverblueAds, and Commission Junction
I've been paid by LeadersMarket, AzoogleAds, CommissionJunction...all checks so I don't have any proof to show...
I get paid by ClickBank, AzoogleAds, AdBrite, Plimus, Commission Junction, and PowWeb .. No need to post proof, everybody knows that i'm trustworthy.. besides, nothing for anyone to gain by posting proof that we get paid..
I seconds GTAce's networks I have received checks from them also. Stick to major networks that others use and your less likely to get ripped off.
Your reputation is proof enough for me Well, there are some who promote obscure networks, where nobody was paid. Having at least one proof in those cases, it could change "no" in "maybe".