Depends on the tone of the conversation.
I might just stay out of it, or I might ask a challenging question, for example on the nature of free will. I myself am not decided on this one; and I find theists have just as much difficulty, but from the opposite perspective.
If the conversation is not about belief per se, but about the imposition of laws comporting to some religion's moral code, (eg gay marriage, abortion, etc) I will argue the matter of human rights and freedoms, and avoid the religious aspect altogether; unless the believers base their arguments on God's command; in which case I raise the Euthyphro dilemma, the 'Cosmological Argument' etc.
Good question; same?