My mare gets really 'excited' when we hit the trails...she is alert, but not goofy, and she loves to have me let her out a couple of notches and trot or canter along for long periods of time in the open air. When we are in the round pen, or other 'arena' type area she is less forward going, and although she does everything she is asked, you can just tell she is just doing it to please me...which to be honest, I would rather have that then a horse that fights it if he doesn't like it.
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.