Unless you're a pure blood Garlean then you have no magical aptitude. It's why they have magitech for their weapons. While probably favoring anyone who's not a pure blood and has a high aptitude for magic put into the military. I kind of feel that it's based on how hardy you are. For instance the reason machinist have those batteries on their hips is because the guild master wants anyone to be able to use their skills. The batteries draw in aether to be used with the more complex stuff. Also the thm guild receptionist doesn't seem to quite have a test, but she has turned away many who don't have the right stuff. That being the right attitude or not enough of an aether/mana pool to draw on depends on the person.