For CMMI, appraisals are supported through the Standard CMMI Appraisal Method for Process Improvement (SCAMPI). You may want to review the Executive Summary section of the SCAMPI Method Definition Document (MDD). These documents describe the method used by lead appraisers and would help you prepare for an appraisal by knowing what to expect.
The full set of SCAMPI appraisal team member (ATM) qualifications is defined in section 1.3.2 “Select Team Members” on page 78 of the MDD PDF.
Basically, every ATM must have completed a licensed Introduction to CMMI course based on version 1.3 or have taken an Intro course based on a prior CMMI version and upgraded to version 1.3 via online upgrade training. Additionally, if the appraisal is using a different CMMI model than that of the prospective ATM's base Intro course, then the individual will also needs to take the respective Supplement courses.
Each appraisal team member must have previously completed a licensed offering of the most recent version of the designated introductory course or upgrade training relating to each and every reference model (e.g., CMMI constellation, or People CMM) that is included in the scope of the appraisal. The experience of the appraisal team as a whole is a decisive factor in the team member choices as well. Ultimately, it is at the discretion of the lead appraiser.
For more information on CMMI appraisals, please see: https://cmmiinstitute.zendesk.com/hc/en-us/categories/204070948-Appraisals