I've always worried about getting emails like that. I can't even see consistency in my OWN art! :P
You, however, totally have a clear style. Your line treatment alone is like a neon sign saying you did it.

But the "if you can't find it then I don't have to show it to you" is a good sign that this person is either a troll, or very confused about how "proof" works. I'm guessing the later.