What does it mean to be "checking someone out"?

This guy I know kept on looking at this girl. He just stare out her and doesn't do anything. He just stare for a long time. I don't know, does he like her? He also smile and laugh when confronted that he likes her? But he never talks to her and always try to avoid talking to her. I mean, they know... show more This guy I know kept on looking at this girl. He just stare out her and doesn't do anything. He just stare for a long time. I don't know, does he like her? He also smile and laugh when confronted that he likes her? But he never talks to her and always try to avoid talking to her. I mean, they know each other for almost 2 to 3 years and not one time did I see them talking not to even say Hi. But hey, they're Asians..... So is he checking her out? What does checking someone out mean? Please answer ....
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