You know that you are a mathematician when:

.. when you see an advertisement that reads "... in partnership with H&O", and the first thought that comes to your mind is: I know what it means H - stands for quaternions, O - stands for octonions and ... What?!?.

I hope it doesn't sound like math is the only thing I think about...

Anyway, it is a good thing that when mathematicians think too much about math they are at least not very dangerous. You just need to remember that if the person you are talking to doesn't know math well enough - it is not a good idea to talk with him about cantor sets, cardinalities, vector spaces, etc... Even if you feel that it is simple and you are sure that you can explain it, hold you self.