I discovered this purely by accident on a first date the other day. We were sitting close to each other on the couch and sort of facing each other. She had used some kind of nice shampoo or conditioner or something, and I was leaning in to smell her hair, and I noticed that she started to lean in too for the first half second because she thought I was going for a kiss, before she realized what I was doing. Obviously led to making out a few seconds later, but it occurred to me that that’s actually a pretty good way to test if she’s ready to cross that threshold. Lean in to smell her hair, and if she’s not into it and leans back or looks away, she’ll realize in less than a second that you weren’t actually going for a kiss, and you can tell from how she reacted how to proceed without any awkwardness.

Just thought I’d share that experience; I hope it helps!