There is a wide range of possibilities to assess an organization's maturity level from a CMM perspective. The People CMM Reference Guide provides (in section 6.4) an overview of the possible assessment methods ranging from very pragmatic (self assessments) to formal SEI authorized assessments using the SCAMPI approach.

In this early SEI document William Hefley and Bill Curtis describe a detailed description of the an assessment.

A useful comparison of general CMM assessment approaches is provided by Richard Waina in part 2 of his CMM article series; the following excerpt gives a summary overview:

 

A CMM self-assessment is used to educate the organization on the model and to begin to identify areas for improvement.