I am finding a lot of confusion about what psychologists are supposed to be doing. For the most part, clinical psychologists diagnose and treat the mental disorders listed in a big book called the DSM-IV. You can find a summary of those disorders on Psychologynet. A better summary has been made available by James Morrison.
There are a lot of disorders listed, but most custody issues don't involve any of them. That is one reason why psychologists should not even be involved in the typical custody dispute.