Besides obvious things like incredibly high stats and strange moves like Spacial Rend on a Magikarp (yes somebody tried pulling that off on me, mostly for fun though.)
If it has unusual stats, looking at the ball used to catch it is another way to tell (unless they are mentally challenged, I don't think anyone would catch a level 2 pidgey with a master ball) also, look at where they met the pokemon. If it says "met ho=oh at route 4" you know it's hacked. some may have a trainer ID of 00000.
First of all, check the Pokeball the pokemon was caught in, if it is a cherish ball, it was hacked. Secondly, cheated pokemon will say "Met at Lv. 0". Thirdly, Cheated pokemon can't be traded on GTS.
Hope this helps.
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