Stage/Phase-Gate Process Audits

When a company defines its product development process or processes that it intends for all personnel to follow, how does it determine that this is in fact happening? How can you determine where there are areas of non-compliance either because project managers and project teams members are overloaded or because of lack of understanding and training? From the standpoint of the stage/phase-gate process, resource management, portfolio planning and performance measurement, how do you ensure reasonable data integrity?