You ask an expert, like a veterinarian, to look at the horse's teeth
Well the most proven way is 2 tell by the teeth and that's the only way I know to tell.
Usually its by their teeth.. Here is a link at the bottom.. But if the horse is only 12 months.. That's rather early to begin breaking him, what do you mean by breaking him? Riding him wise?
You can tell by their teeth in three ways: The angle of the way the top/bottom teeth meet, the cups inside the teeth, and the groove on the last incisor. Best to have an experienced farrier or vet perform this age check for you because it can be confusing due to all three parts to take into consideration.
The way to check a horses age is have the vet look at his teeth and he can check the rings and give you an approx age.
You should never break a horse to ride before it is at lease 2 years old!!!!