How does judith loftus figure out hucks disguise?

Answer 1

Huck catches himslef in a lie when he first tells Judith his name is Sarah then he changes it to Mary.

Answer 2

Judith Loftus becomes suspicious of Huck's disguise because of his inability to act like a girl convincingly. She notices his mannerisms, speech patterns, and physical movements are not that of a girl, leading her to conclude that he is not who he claims to be.