The ability to detect transfolk, especially when the identified person is passing. Generally one tunes in to anatomical cues such as hand structure, brow ridge, adam's apple, and hips.
> A: Wow, she is gorgeous. Look at those lips...
> B: Look at those hands! Girl is setting off my trans-dar. Are you sure you're ok with that?
> A: Why not? She is really something. And besides, I have a few things to learn.