So, after thinking about it, Mayt already knows fake Karn is fake. Seeing the blood will just confirm fake Karn did something

Reasoning: At the train station, Mayt felt how much magic Karn had which is how he knew Karn want a cat, but not by the normal way people sense magic. He used his energy control magic to "sense" it. Right now, he's basically feeling up fake Karn, giving him a great "sense" of how much power he has stored. Now he just wants to see what edo-Trinity found so he can compare and see if fake Karn did something to edo-Karn.

However, the guess as to the identity is probably going to be wrong.