....because if he is, he's probably interested.
What am I talking about? Well, I've never heard of the search engine before, but somehow I'm the number one result for this.
Well, I'm no expert, but I'd pay attention to the following: When you talk to him, does he stare you in the eyes when you're talking, but look away when he is talking? Especially if he's talking about himself? Does he carefully clutch something nearby, as if restraining himself from coming closer? If he's walking by and he suddenly spies you, does he suddenly slow down? Does he seem to be facing in your general direction when you see him (an indicator that he's already seen you)? After a conversation with you ends and he's walking away, is there suddenly a little more spring in his step? Does he seem to be always trying to make you laugh? If you listen in to his conversations with others, and he's generally witty and articulate, does he seem to grope for words and get tongue-tied when he talks directly to you?
Those might be good indicators of interest. Years of people-watching pays off, once in a while!
(If you're wondering how to tell if she is interested, well -- I haven't a clue. It's a total mystery.)