Two more non-trustworthy indicators:

• Ask the person in question which of the several ridiculous ideas they reject they find least ridiculous - for example "Which do you think is more likely to be true - astrology, or UFOs?" I've found people trying to signal affiliation have a hard time with this sort of question and will even be flustered by it, saying something along the lines of "They're both stupid" or "Is this some sort of trick to make me sound like I believe a crazy idea?". A rationalist will say something more like "Well, I don't believe either, but UFOs at least make sense with our idea of the universe, whereas astrology is just plain crazytalk" (or ze may refuse to answer on the grounds that you're wasting zir time; it's not a perfect test).

• Observe the circumstances in which the person involved brings up the belief. If they just go to atheist forums and say "Man, those religious people sure are stupid," higher probability of signaller. If they actively talk to religious people, try to use atheism as a starting point for building new ideas, and don't bring it up much when it's not relevant, higher probability they believe it for the right reasons.