When I'm hugging a guy who isn't interested, he'll usually hug from the side with as little body contact as possible - usually with a pat on the back during it. When a guy wants to be with me, its a full body contact hug with a kiss of some sort (unless he's gay - I had a gay friend once who said he liked to hug close to me because of the 'big comfy boobs'

)
I guess in your case you'd have to look at his other body language too. Is he looking into your eyes? Is he positioning himself so that he is 'open' to you (no crossed arms over chest or looking away a lot)?