With Lacey, I discovered what she loves to do (being a trail pony) basically because she kinda forced me to. She kept freaking out in every english saddle I tried on her (I originally bought her with the intention of doing dressage and things like that) so one day I decided to try her in a western saddle. As soon as I got that saddle on her, she went perfectly.
Then, I took her to a camp last summer and did a lot of trail riding. I had previously thought that she was a spaz on the trail and that that was not something she was interested in. About the time a flock of 100 or more quail cme bursting out of the trees right next to her, with no spooking on her part, I realized that she really does like being on the trail. Now I've been riding her more extensively on the trail and boy, she is the bravest, most curious, fun to ride on trails horse I have ever ridden. Whenever the trailer is out, she starts bouncing around and waving her head at it like "people are going somewhere! I wanna go!" so I'm pretty sure she enjoys being on the trail.
Basically, listen to your horse, when he finds something that he dearly loves to do, you'll be able to tell. Lacey is not built to be a great trail horse (tiny legs compared to her body, really rocky trot, etc) but she really is. So, just try out lots of stuff with him and he'll let you know. =)
Fabio - 13 year old Arab/QH gelding
Hazel - 14 year old Angora goat
Atticus - 4 year old LaMancha/Alpine cross goat
Rest peacefully, Lacey.