It all depends. Is it just a friendly look, a "how you doin'?" look, etc.
You can normally tell the difference between a guy just glancing at you, a looking at you with attraction, and a guy thinking if he can twist you like a pretzel. Typically it's left to the discretion of the two. If either find the other attractive, and if either have the courage to talk to one another.
At least smile, that goes for any person, you don't want to put out a bad vibe right? lol and then go from there, depending how they react to your smile. Just say "hey" or "hello". Maybe they'll say something to you (hopefully something nice and appropriate).