How do we know what's in Montegomery's mind?

The defence lawyer in the gruesome case of Lisa Montgomery has made a diminished responsibility plea based on the claim that she is mentally ill on account of a history of child abuse. The prosecutors claim that she is making it all up.

How do we know? If you lean towards a Logical Positivist outlook then only empirical evidence can be taken as a claim to truth. The arch exponent of this line of thinking was B.F.Skinner noted for his use of behavioursim as the only way to conduct psychology. In other words he claimed that psycho-analysis was pseudo-science as it speculated on the workings of the mind rather than restricting itself to the observable.

The prosectors will be taking this line...and, unpalatable as it might seem...what if the defence are correct?