If you are a known atheist or a member of an atheist group, then you will occasionally find yourself dealing with religious spies. That is, religious people who will either pretend to be your friend or join your group in order to observe you. Calling them spies may sound paranoid and it may give their activities more dignity than they deserve but that is essentially what such people are and serves as a useful description.
If they are honest, they might even admit, in a way, why they are there. "Just curious" they might say. In many cases, however, you can expect no such decent act of openness.
In either case, that person will be probably be there to confirm his or her prejudices and gain ammunition to use against atheists, or maybe even you in particular if he or she (or a friend) has a personal ax to grind. Don't forget this little horror story and how religion and the religious can be used as a weapon.
There are more than enough resources in libraries and on the internet these days for anyone who is "just curious" about atheism to learn all he or she needs to know. If the person wishes to observe actual flesh and blood atheists in their casual and relatively unguarded interactions, then the person is probably still harboring bigotry about atheists. That person probably wants to see if we are as evil as she has been told.
Consequently, the first thing to remember is that a religious spy will give anything you say or do a bad interpretation, so minimize your contact with that person and be careful when in his or her presence.
The best approach is to shift the focus onto that person, to put him on the defensive and thus focused on himself and not you. This will also give you a chance to determine just what the person is up to. Ask questions about his opinions and why he is there. Observe his demeanor. If you see nervousness or evasiveness or an obvious attempt to cover it up, then you will know why he is there. Come right out and "jokingly" accuse him of being there just to confirm his prejudices and look for dirt to use against atheists and see how he reacts. But, say it with a smile, so as not to seem hostile and let him know that his activities are obvious, silly and barely worth your concern.
Often they are there to gather information on atheists' thinking and the arguments that we use against religion. I wouldn't worry too much about whether they hear our arguments. Our position is logically valid and sound; their position isn't. See this post for a summary. There is nothing they can know in advance that will change that fact. The best they can hope for is to camouflage one of their old arguments sufficiently that we appear flustered when presented with it.
In my experience, the religious only have a handful of arguments and that's all they ever had. They have been making them for centuries. They just keep re-phrasing them and dressing them up differently. The key to debating theists is to listen to what they are saying and compare it with past experience until you recognize which pig wearing a new shade of lipstick is being presented to you. Then treat it like the pig it is.