He looks at you when you're not looking

This is the one most women miss, because the moment you turn around to check, the expression changes. But catch him in those unguarded seconds — when you're talking to someone else at the table, when you're reading, when you've stepped away to take a call — and you'll see something his face doesn't show when he knows you're watching. The expression is softer. The attention is total. There's no performance in it because he doesn't think he has an audience. That unguarded look is the real answer to the question you've been asking. The face people arrange for others is a curated version; the face in the quiet moment between glances is the honest one. It's why the poets kept writing about it — there is something unmistakable about the way a person looks at someone they're falling for when they think no one is watching. You may only catch it once. That will be enough.