What are some ways to tell if someone is interested in you if they are not in a position to be direct?

(And no, I’m pretty sure they’re not married…) Here’s the deal; I have become rather smitten with a former professor of mine. (He’s four years younger than me, so there’s nothing creepy going on there.) Since the new semester started, we have run into each other any number of times and talked on a number of occasions, but I know he has to be careful about appearances while we are on campus. I am hopelessly shy so the chances of my “asking him out for coffee”, as my friends all suggest, are slim to none. I am just wondering what things I should look for to that might indicate he feels the same way…