Checking People Out

I was hanging out with Alvin yesterday and after eating Slurp, which is like super cheap Chinese food, he saw one of his friends and started talking to him on the street. Then Alvin was like "that girl is totally checking me out" and I was like "uh ... how would you know" and he said "because she's looking at me and smiling."

Now, I'm not an expert on anything, much less the ways humans flirt with one another but that sounds like a pretty contestable line of reasoning. There are tons of reasons why a girl would be smiling at a guy without the intention of flirting. Off the top of my head:

1) Her friend was making fun of you and she was smiling and laughing at you
2) You remind her of something funny and she was laughing at you
3) She's smiling at something else and is just casually glancing away, but coincidentally in your direction
4) She's actually checking someone else out across the street but you're assuming it's you
5) She's a happy person who smiles. And happens to look at things with her eyes

I don't know but when I'm checking someone out on the street, I would never bother to smile at them. First, that's super awkward. Second, it's not like the person is going to stop on the street and start talking to you. We don't live in the country. We live in urban/suburban cities where people keep to themselves and are constantly rushing around. Third, it takes a lot of effort to actually smile, and when you're walking around smiling to yourself, you look like you're on drugs. If I were actually checking someone out, I would just pretend to happen to turn my head in their direction as I pass by them.