Generally speaking, Smogon is the place you don't have to worry about that. Most of us use AR to clone pokemon here, but there is rigorous moderation concerning hacks and the like.
You might not trust me from a hole in the wall, but there are numerous pokemon in my thread that have been gotten from well-trusted people on Smogon. For example, MattJ is a popular RNGer, and has many references on this forum. If you trade for something with an OT of MattJ, and an ID that matches his, from this forum you can be almost certain that it's legit (I say "almost certain" in the same way that I mean "it is logically possible that Obama is a nazi space alien socialist" - IE, not realistically possible.)