Right?! She should be able to determine if someone is being truthful.
Not sure how she does that. The State had Letecia's own lies and 23,056 different stories to show us, so we know she's lying.
But in general, both physicians and anthropologists are trained to listen and to have as a first order premise, that the! patient/client/subject is NOT lying. If a person lies, we have to have evidence of it (such as another person saying the opposite, in which case, we need still more evidence to know who the liar is)
Of course, outlandish lies are in a slightly different category. But if one is interviewing, say, a schizoid patient and they say they pray to some invisible deity and get results we don't treat that as a lie. It's a religious belief. It's not even a delusion because so many people have the same belief.
If a person has a peculiar belief (a blue Leprechaun is giving them the winning lottery numbers) that can both 1) be disproved (they aren't the winning numbers) AND is not widely shared (we all know that either 1) Leprechauns are green OR 2) they do not exist in the visible world and do not speak to people, then we say it's an unusual or eccentric belief.
Typically, psychotherapists establish lying by having several sessions over a couple of months, in which they are able to observe the patient lying directly to them about a matter involving the therapist (such as changing their story, after therapists has taken notes documenting a different story). Same in field anthropology.
And at trial, the Jury gets to decide who to believe.
IMO.