I'm not good with advice, but I'll try what I can to not get you killed. (serious face)
Anyway, there's no sure-fire way to know if your friend is a furry, but you can try to look for clues that may help in you guessing if he's a furry.
1. Does he like to draw? If so, why not ask him if you can see some of his drawings. If there's some anthropomorphic stuff in there, or if he's being protective about some of his drawings, he could be a furry, and not want anyone to know.
2. I wouldn't do this right off the bat, but you can try to casually bring up the subject of furries, and talk about the fandom (casually, but not enough to make him think you're a furry), and perhaps he might spill the beans.
On the other hand, it's not a majorly big deal if he knows (of course, I'm not you, so I don't know for sure how urgent it is), but if he's a true friend, he won't tell anyone and accept you for who you are, furry or not. That is the ultimate friendship test. I told my friends I was a furry, and none of them thought any different for it. Heck, I even told them I wanted to go to Anthrocon and throughly explained it to them, and they still treated me the same. That shows how good your friends are. If you really feel you should, tell him. He's your friend, and if he respects you, he won't tell anybody or make fun of you.
I'm sorry, I'm just so bad at giving advice. It's not my forte.