Crypto-Atheists

The idea scares the hell out of me: namely, that
one of the people in my congregation is an Atheist
masquerading as a Christian for social advantage
(or sexual advantage, for that matter).

Is there some tried and tested method that allows me
to determine whether or not a person is a true believer?

Test there intelligence... If they are able to rationalize decisions you have got your self an Atheist.