How can you tell if someone is an atheist?

Answer 1

Generally, there are no obvious signs, other than a lack of belief in any gods. Atheists have no secret handshake, uniform, horns, cloven hoods or large red "A"s tatooed on their forheads. They generally don't quiz people about their belifs or engage in random arguements about the rightness of their position with strangers as past history often sees this as a source of anger and fear among the faith community. They seldo use phrases like "God Bless!" or "All we can do now is pray for him!" They are active in charity, boy scouts, politics, schools, civic affairs, clean-up days. They do have families and friends, An Atheist could be anyone.

Answer 2

Only by asking them. Even then they are under no compulsion to tell you the truth. Atheists have no physical external manifestations, accents, secret handshakes etc.