First of all, you can judge by his origins, his breed and the respect /temper of the horse.
There are many factors that make the height vary.
It depends of the level of the rider (a beginner won't go as far as a professional with the same horse), the age of the horse, his dressage level.
You can see if your horse has difficulties when a make big jumps, make "noise" while jumping, breathe too much and want to stop in front of the fence (or don't respect the height and make a lots of faults).
To have an idea of what your horse can jump, you can make him jump in liberty ans see his limit but know that with a rider on his back you'll have to remove 20cm from this maximum height.
Don't hesitate to ask a professional (monitor or breeder) to have more informations if you try to buy a horse.
I hope i could help you :)