...which could always be an auditory illusion but then she could also just not be a Lawful being in the first place if they're lying about that. At some point you have to notice that these eight million doomy possibilities are all highly conditionally dependent on each other, meaning that the world in which they're all false has a decent-enough probability.
Keltham almost starts to sign, before he remembers that he does have a tool for being a little less uncertain, possibly given to him by his god (if indeed that is actually a thing, as others told him, and books seemed to mention), even if it's a tool that seems like it could obviously be defeated in some ways. "I have a first-circle spell which, I am told, enforces that truth be told, and shows a symbol to prove this has come into effect; I apologize if this is an insulting question, but is it acceptable that I cast that spell on you, and ask you only if you are indeed a Lawful being, and if you've lied to me at any point before?"