If a guy looks at you from your head to your toes slowly does that mean he is checking you out? How should one react?

Today, a guy who was standing next to me, took a step back and slowly looked at me from the top down. When I asked him what's up, he just brushed it off by saying that he saw something on my skirt. Is it safe to assume that he likes me? I also have realized that I didn't react in any way. For some reason I felt uncomfortable in the moment. Maybe it was because it was slow. How can I react in the future if it happens again to test the waters to see if the feeling is mutual?
If a guy looks at you from your head to your toes slowly does that mean he is checking you out? How should one react?
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