I'd assume that while a person's humours can fluctuate, they'd have a baseline that isn't the same as everyone else. A weighting off perfect balance.
Detecting that might require a more potent spell than one that just detects current humour balance. Probably not much more potent (InCo 10 or 15), but probably enough that you'd want to actually invent the spell rather than just popping it off spontaneously.