Zoroark cannot speak, so if it did disguise itself as a person, then I'm guessing it wouldn't be too hard to figure it all out. Zoroark loses its illusion once it gets damaged, anyway, so if it ever got hurt—even a little—then it would transform itself back to its normal form immediately.
If that event ever ended up occurring, though... then I'm not exactly sure what I'd do. xD" I would feel quite surprised about it, that's for sure. Well, "surprised" probably wouldn't be the best way to describe the situation, but y'know. If this Zoroark is a member of my party, then I would release it as soon as I can and try my best to completely erase it from my memory. If it's not in my team, and I'm not associated with it in any way, then I'd call out one of my Pokémon and have it scare Zoroark away.