Do any of you know a way to actually tell whether a Pokémon acquired is legit or just a cheated/copied one?

Reason is I got traded a Mew recently. It shows the correct ID and original trainer name (when checking it with various websites), also has the correct moves, the IV are good average but nothing extraordinary (according to the guy who can tell you that in Platinum). So does all that mean it is a legit Mew that was given out at an event or could all that still be cheated by some means unknown to me?