How do we know the company is CMMI Level 5 or not? Their company is not listed on the SEI - PARS system (public list of CMMI Appraisal Results).
Many companies list CMMI MLx in their marketing literature, or in proposals. There is nothing wrong with this - they should be proud of it - except in the case where they have not had an appraisal conducted within the past three years.
As you know, PARS is the website (sas.sei.cmu.edu/pars) where CMMI Appraisal results are displayed. It's a public site, and a company must initial their Appraisal Disclosure Statement (ADS) to signify they wish to be listed on the site. Most do, but I have had clients that did not want to due to propietary information they did not wish to share, or they were working on a sensitive DOD project.
The official record of the appraisal is the ADS, not the website listing, so if you need to confirm that a company is indeed ML5, and that their appraisal is current, you can ask them to show you their ADS. If they are not willing to do that . . . . well, you can make your own conclusions.