Out of curiousity, what are subtle signs a guy is checking you out or finds you attractive (as complete strangers)?

I rarely go to public settings since due a health condition and covid I couldn't go out literally for about 3 years. Today I sat down for the first time at a mall lunch and noticed that while there were guys focused on their food, or companions, others were staring while eating from a distance. I don't know if I'm just more sensible to this due the social dynamic I was living but I wonder how can one tell is a guy is checking you out as a stranger?

Out of curiousity, what are subtle signs a guy is checking you out or finds you attractive (as complete strangers)?
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