we don't go around challenging self-identified Christians to provide evidence of their belief.

Lol you've not spent much time in churches if you think that. Everywhere I've been is rife with these types of people. It tends to be a frowned upon behavior by most since it ends up turning into a purity spirals, but you'll find all sort of judgemental types finding evidence for why someone isn't "a real christian" based on everything from their fashion choices to the movies they watch.

My favorite is the ones that rail on "real christians don't drink alcohol" they're pretty easy to embarrass publicly if you ask them what Jesus' first miracle was.

If drinking alcohol was bad why did Jesus supply a party with more alcohol after the alcohol had been drunk and everyone was sloshed already? Guess Jesus is a bad influence in your point of view eh?

Usually triggers them quite funny.