We don't really see it when they are young. We can see that they're good but they can me good for amateur classes. When a horse is 6 or 7 years old, we can see if he will be able to jump Grand Prix.
He needs to have energy, a good mind, a good jumping style and technique. It also has to seem easy for him. But above all, he has do want it.
They do circuits for 4, 5 and 6 years old horses. Some horses show their quality quite late, whereas others are excellent at 4 years old and less good after.
Their health is also crucial, they're not all equal regarding their health, since their birth. Showjumping requires a perfect health, otherwise they can have problems with the accumulation of efforts. It's so rare to have horses who have the talent, the mind, and the health, that these horses are worth a fortune.
That's why it's really important to have a lot of horses, breeders who breed a lot of horses have more chances to have a champion, even if a breeder with only one mare can have the chance to have a 5* foal!