Here's the simplest, shortest answer:

A guy who looks for opportunities to be around a woman is sexually attracted to her. If a guy is willing to set aside time to be in a woman's company, he is attracted to her.

"But couldn't he just be looking for friendship?" Not unless he's getting his need for intimacy met somewhere else. And, if he does have a woman who meets that need, then you got to wonder: why is he spending time with his female-friend that he could be spending with his female-partner? Suppose my guy says to me: "Honey, I'll be a bit late getting home tonight because I'm stopping at the diner for coffee with my friend, Mary." My response will be: "Don't bother coming home at all."

Here's another scenario: My guy says this - "Honey, would you mind if I invite Mary to have dinner with us tomorrow night? She's an old friend and her house just burned down." My response will be: "Sure. Does she have a place to sleep?" Couples socialize "as couples."

Naturally, men who work with me, or attend class with me, are also going to talk to me. They may sit with me in the cafeteria, at work or at school. But, if one of them asks me, "So what do you do with your weekends?," I'm not going to figure that the guy is just taking a survey.

Another clue is the desire to be helpful. If a guy says he could loan me a tool for a project I'm working on, that's being friendly. If the guy says he'ld be glad to come over on his day off and do some repair work on my car or on my home, then that guy finds me attractive . . . or . . . he needs to get a life.