I couldnt say if magical aptitude is a direct correlation to how much aether a person has but in many points in the story we see people talking about how much aether a person has. The scions also have their goggles which allow them to see aether directly and Y'shtola is able to "see" aether itself as well. I wouldn't be at all surprised if the thaumaturges and arcanists had a ritual or spell to cast that would give an indication of a persons power. We also know from the Alchemist quests that ink can react to a persons aether so it wouldn't be a stretch for them to have a potion/ink that would change depending on how strong a persons aether is.

My view has been that aether is the battery that drives all the spell craft and its only a persons training or temperament that dictates what they're good at; some people like their cute carbuncles, others like to incinerate things, and some just want to stone people.