Have you spoken to him about it?
If you’re compulsively obsessing about something the best is to catch yourself and tell yourself to stop thinking about it, the more you do, the bigger it becomes. You’re overestimating the risk. What is him checking someone out actually doing or mean? Probably nothing realistically.
However, it might be helpful to understand what about is bothers you? Has it always bothered you, has he always done it? Could you be scared he’s looking for someone else, doesn’t pay you enough attention, comparing…? Can you break it down to get to the heart of the problem and address how you can overcome that.
It may be that unless you can address how it’s making you feel, he could stop looking and you’d find or suspect another behaviour in him that would make you feel the same.
Incidently I think most everyone does it, not just men. We all look at others, check out someone attractive, male or female. Not because we want to sleep with them, it just draws the eye.