Clever modelers will leave a specific mark on the model that is so insignificant its clever. Like the exact posistion (y, x, z)of a vertex in a mesh.
Then again you just genrelly know wether the model was stolen or not. If a guy says no word about how he came out with 20 diffrent tank models and not really any explanation then you know.
Idk, the files that someone would need to use to skin and test the model in game.
So 3ds max file, .RGM pretty much.