Look him in the eyes

It doesn't matter who he is, if he's worth dating he'll notice this signal. Give an "open" face and look him in the eyes. If he reciprocates, he'll probably glance down to the corner of his eye after a few seconds. That show's he's affected, whether or not he looks back after that. Usually, though, it will be hard to ignore. Let him say the opening words (or word, it could be as simple as "hi"), then respond. Don't go straight into setting up a date; the goal is to lay the foundation for your next encounter. But try to make a small, fun conversation before you leave, so it doesn't drop awkwardly. Make sure his last memory of you is a smile.

Negative Signals:
In that situation, the guy won't be ble to look unconcerned unless he IS unconcerned. It'll be obvious.
Watch his eyebrows. If they furrow, then he's probably not interested.
If he looks straight down, he probably likes you to some degree, but he's unwilling to enter a relationship.