Do you think he's interested in me?

I see this guy who works in a supermarket I go to, whenever he sees me we have eye contact and he smiles.. does this mean he's interested? I don't think he's just being nice because he works in a supermarket, like someone has said The thing that keeps playing on my mind is that he used to go to my school a few years ago, so he could just be being friendly.. but then I think he wouldn't do what he does if he was?