1. How loud do its proponents talk? If they're shouting, that doesn't make what they say truer. On the contrary: There's generally an inverse relationship between decibels and truth. Besides, do you really enjoy being preached at?
2. Do they know what God wants/thinks/intends? If so, either they are God or they think they are God. That's called heresy if you're religious, and psychosis if you're not.
3. Are they obsessed with sex? If they're threatened by women or are LGBT-phobic, there's weird sexual stuff going on. If you're similarly threatened and phobic, Westboro Baptist Church or Mars Hill Church will provide a home for your penis.
4. Do they have good music? Christians might have this one beat (Bach's Mass in B minor, gospel music...), but if you've never heard Pakistan's Nusrat Fateh Ali Khan, you have an ecstatic Sufi feast in store.
5. Talking of feasts, do they have good food? Communion wafer, anyone? At least Jews have matzo-ball soup and those four glasses of wine at Passover. And Muslims get to dine on fatted lamb at Eid al-Adha—but winelessly.
6. Do they cite chapter and verse at you? This is the primo tactic of fundamentalists: cherry-picked quotes, out of context. Try tossing this one back at them: "The letter kills, but the spirit gives life." (2 Corinthians 3:6.)
7. Do they have any idea what "metaphor" means? If not, gently suggest they sign up for English Literature 101—no, demand it. Do not put up with literalism.
8. Are they into social justice? That's the essential subtext of both the Bible and the Koran: social and economic protest against corrupt elites. The Big Three monotheisms began as the Occupy movements of the ancient Middle East. Where do you think Marx got his ideas from?
9. Do they insist on your swearing belief/loyalty/obedience? If they lack a sense of mystery and claim to have all the answers, run like hell. That's not faith, that's dogma.
10. Are they into joy? Do they celebrate life—in this world, not a next one? Do they make you want to laugh, cry, hug, dance, stay up all night and watch the sunrise? Do they make you happy and grateful and goddamn humbled by this strange thing we call existence? A++ if they do.