With Jimi, who barbers, I tell either by what is on the cage floor or remaining on him. It's as if you snipped the feather off low in a "V"in various sizes.How can you tell if he's barbering or plucking?
He is never actually bald, there is just a short stub of feather plus the grey under-down there.
When occasionally I see a full feather on the floor there is always soon the opposite side's corresponding feather like a normal molt.
I don't know what to think if you never see anything. Not even teeny tiny feather pieces? I have no ideas for that mystery!
I know if a feather plucked often it might not regrow, but I don't know enough about your situation to make a guess.