Proposed litmus test: infanticide.

General cultural norms label this practice as horrific, and most people's gut reactions concur. But a good chunk of rationality is separating emotions from logic. Once you've used atheism to eliminate a soul, and humans are "just" meat machines, and abortion is an ok if perhaps regrettable practice ... well, scientifically, there just isn't all that much difference between a fetus a couple months before birth, and an infant a couple of months after.

This doesn't argue that infants have zero value, but instead that they should be treated more like property or perhaps like pets (rather than like adult citizens). Don't unnecessarily cause them to suffer, but on the other hand you can choose to euthanize your own, if you wish, with no criminal consequences.

Get one of your friends who claims to be a rationalist. See if they can argue passionately in favor of infanticide.