His pupils dilate when he looks at you

This is the involuntary one — the cue that is genuinely impossible to fake, because it's controlled by the autonomic nervous system rather than the conscious mind. Pupils widen with genuine interest, with positive emotional arousal, with the presence of something the brain registers as desirable. It happens regardless of lighting, which means you can separate it from the normal pupil response to dim environments. Most people don't consciously notice dilated pupils, but their unconscious does — which is part of why being looked at by someone interested feels physically different from being looked at by someone neutral. The body recognizes the signal even when the mind doesn't. To check it, you need to be close enough and in consistent enough lighting to see a change. It's not always accessible, but when you can see it, it is the cleanest data point of all — the nervous system reporting directly, with no social editing in between.