It’s usually pretty easy. Are they engaged? Are they responding quickly? Are they actually hearing what you’re saying? Engagement means their mind is paying full attention to the activity at hand. And the person it’s with.
Guys can fake engagement if they just want to get laid. In this case you want to go deeper and start ascertaining compatibility. Ask questions that don’t have easy answers. Guys will generally have one of two approaches to answering such questions. They’ll either try to give you something you want to hear, or they’ll answer honestly, not really worried whether it means that’ll be a deal-breaker or not.
If their answers seem to all be too “perfect”, then he just might be leading you on.