In my experience in the game so far, there's no way to actually "see" the hitboxes. However, they certainly seem to cover the entirety of the characters, and possibly a little bit more. I play Hanzo almost exclusively, and I've gotten some strange kills with him. I've gotten a few kills by hitting people in their toes. There was one, which I still don't exactly understand, where I wall climbed and fired a fully charged arrow downward when I got to the top. I did not know there was anyone up there, but suddenly McCree went running by/near/under me, and he just died. I almost didn't even know he was there until he dropped dead. I wish I had a FRAPS recording of that to analyze it more, but I really didn't think I had hit him and suddenly I had.