Firstly work over ground poles... evaluate how they move over them, how they pick up their feet. Then move on to free-jumping. Evaluate how they maneuver the jump, and especially how they take to it. Do they enjoy it? Does it seem tedious to them? The more you do it, the more comfortable they become with it, and the more you will be able to see their emotions regarding it. A horse first learning to jump is probably going to be a little clumsy, they are learning where their feet are and how to maneuver this new obstacle. But the more you free-jump them, and help them with grid work/etc, the more you will see their striding, jump, and bascule improve. If you don't see a lot of improvement, then chances are they aren't very good jumping prospects.
However, you must give every horse the chance to succeed. Start them correctly over fences, with grids, ground poles, etc, and give them the opportunity to learn and succeed. Then evaluate whether they are a prospect or not based on how they've come along and taken to it.
A lot of faults/issues you see in young horses, or green horses over fences, can be fixed with exercises to aid the horse in improvement. However, there are some tendencies that are very hard to fix, and can be dangerous. Such as hanging a leg, laziness when jumping. The horse has to enjoy the task, and want to do it well, to be successful. :)
Also, though, confirmation definitely plays a part in how much ability he/she will have.