George couldn't see Hobbes, but he can see lies/truth so if even Calvin mentioned Hobbes or talked about him, he'd get that Calvin was telling the truth about there being a Hobbes.
Whether or not that would make George decide Calvin is crazy is another story. :D
Re: I Might Be Able to See Hobbes? - I'm X!
Whether or not that would make George decide Calvin is crazy is another story. :D