He grooms himself in your presence

Touching the hair. Straightening the collar. Adjusting the watch or the sleeve. Running a hand across the jaw. These are preening behaviors, and they happen when the brain registers an audience it wants to impress. The preening is usually unconscious — he isn't thinking 'I should look better right now'; he's just suddenly, obscurely aware that his appearance matters in a way it didn't a moment ago. Notice when the grooming happens — it's usually within seconds of you appearing, or after a compliment, or just before he moves toward you to say something. The timing is the tell. Someone who adjusts their shirt when their boss walks by is responding to authority; someone who touches their hair when you walk in is responding to attraction. The same behavior, very different triggers. Pay attention to the gesture, but pay more attention to what set it off.