I´m currently trading Pokémon and I don´t want to be cheated.
Pokeball: If you get a blastoise that comes in any ball besides pokeball then it is fake because blastoise can only be available in a pokeball
If your pokemon knows a move that it cannot learn (besides egg moves) then it is fake. So if your blastoise knew fire blast it is fake but if it knows hydro pump then it is real.
Look where it is caught. I once got a lv 100 shiny pokemon but it said it was caught at route one at that level. If it says anything like that then it is also fake.
Look at the stats. Go here to view all the pokemon. Click on the name to view there stats. If the pokemon you got has stats that are higher then it is also fake.
Hope I helped!
Also with black and white if a pokemon is not legit it won't let it be traded, this also happens with event pokemon. it might say one of the pokemon in the other players party cannot be trade or something along those lines.