Bogus test, used now and then, reveals true nature? (4)
I believe the answer is:
outs
'reveals true nature?' is the definition.
'outs' can be an answer for 'reveals' (outing is a kind of revealing). I am not sure about the 'true nature?' bit.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for outs that I've seen before include "Forces to leave state of concealment, exposes" , "Publicly exposes a person's secret nature" , "Reveals" .)