My Appy that I had last year was like that; if he didn't like something that you wanted him to do, he would argue about it, and he was one of those horses that DOES not take his or his rider's safety into account, so there were many times that we were both in danger of being hurt because of his "I don't wanna" antics. He still had to do what I was asking, don't get me wrong, but man there were days that he definitely reminded me how powerful even an 800 lb horse can be!
My horse prior to that, was another trail bum...he was a fantastic trial horse, although, like the mare I have now, he certainly tried his hardest, even if we did something he wasn't overly crazy about! He was the kind of horse who would walk through ANY kind of trail, and would go over brush, and everything, no matter how much it rubbed on his belly, or snapped at his legs.
I think for the most part you can tell if a horse enjoys his job by his body language; obviously if a horse is always sluggish, pins his ears, or is constantly arguing with you, he may not be suited for that particular job, so you may have to switch things up for him.