It never shows up on any trait list and you don't get any notice of the child having it nor any choice between what traits you get, thus 'hidden traits'.
Therefore, in order to check your sim for this, you have to put up 'testing facilities' as I personally call them, i.e. set up stuff in rooms that test without doubt if they have that trait.
Examples (the ones I've used and can verify effective):
No messes trait: Have your sim prepare 15 dishes in a row, specifically those dishes involving using the counter. If the counter hasn't become dirty after this, you have with a clearly reasonable chance a No messes sim.
Pyromaniac trait: Click on the sim, if it says something like 'set fire to something nearby', he/she is a pyromaniac. Otherwise, he/she is 100% not.
Fire proof: Set up a fire in your kitchen by leaving something boiling on the stove (do not use the most expensive stove, nor a stove enhanced with the fire proof upgrade) and enclose your sim first in a fence or similar. If you're having trouble letting the fire spread, place some mats on the floor. The fire should then spread all around the enclosed area. Keep your sim from extinguishing the fires by cancelling those actions when they appear. Now, if your sim still lives after the inferno, he/she is fire proof; if the sim dies, just normal.
There are probably other, similar techniques, but these are the hidden traits I've tried and normally use.
