Yeah roll to see if you can see the Onta at least. Whats an RPG without Mages?? Would be a real shame if you don't have a real option to become one.
All that See aren't Schoolmen. I think it has a lot to do with being recruited at a young enough age. Those who come from High Ainon from a big city like Carythusal or Aoknyesus (sorry spelling), or from a Norsi fishing village/city, would have a decent chance at being discovered and trained from a young age. Those in Sumna might be recruited to that sect of schoolmen hunters, the Luminaries, Momnem recruited to the Saik.
If you aren't from a major city, there is almost 0 chance you are a schoolmen, or at the very least you are way behind learning. Also, Caste-Nobles are less likley since its taboo.
What I'm getting at is I think that there are far more people who can see the Onta than we are lead to believe, but depending on where you are born, your upbringing, religion, etc., you start limiting the number of people really quick. I think limiting it to 1%, or another such flat rate, seems unfair and unrealistic. Through the character generation process you will decide many things about your character, some are player choices, some are chance rolls (like family status I know for sure). It will be important early on to roll to see if you can see the Onta, but the final roll should be left until later on when you know more about your character. We could develop a small table, based on the factors I described above, which will give you your success range when you roll that D100.
A nobleman from Sumna will have about a 1% chance, but a Nobel from Ainoi might make it. A caste menial in Norsi would have a greater chance still.
Maybe that is too convoluted, I don't know

, but at least then the player can make choices and have some options rather than a flat No or low/impossible odds.