You can check the trainer ID number, if it says something like 00000 or 12345, then that is sign that it could be hacked. though not guaranteed, look at what pokemon the person wants for it. Think about it, if I just worked hard to legitimately catch some really rare pokemon, do you think I would trade it for something like a dragonite or garchomp? I could easily get those in most games, so I would probably trade it for a really rare pokemon. If it's hacked, they could easily make another, so they have lower standards for what they are willing to trade for it.