Moody and the boggart

[Regina]

No: HPFGUIDX 77783

Question: If nobody knows what a boggart looks like when it's alone 
(according to Lupin in Book 3), how could Moody confirm that there 
was a boggart in the desk at 12 Grimmauld Place?

Related question: If the boggart knew Moody was looking at it through 
the ceiling and desk, and therefore assumed the shape of his worst 
fear, what do you think it was? Crouch Jr? or something more abstract?

--Regina