The psychological test can detect personality disorder traits and the evaluation can shed light onto what may be some of her "issues." Even if she seems apprehensive to answer the questions, that too would be noted by the psychologist.
If the assessment is a part of the custody determination, she'd have to go through it.
There are going to be other factors that would be taken under consideration when making the custody decision.
Not sure how old your daughters are but some clinicians also assess the children and at times can get an idea of what is going on in regard to the family dynamics.
If your wife has had a history of aggressive behaviors and there is a record of it, your attorney most likely will bring this in court to the judges attention as a concern.
A good clinician should be able to see through her.