Hi Nali. The MMPI--2 is a very, very reliable and valid instrument for measuring psychopathy, and it is as psychometrically sound as they come. It has internal constructs designed to "catch" exaggeration, over reporting and under-reporting--basically it is designed to identify those who lie on the test. And while there is no instrument out there that is perfect, this is the absolute best their is to offer when a clinician suspects psychopathy.
However, you are absolutely correct, any Clinician worth his or her salt (and by ethical standards) MUST take into account collateral reporting of family, friends, co-workers, etc..., in order to diagnose an individual appropriately.