You could start here, follow up there, read a book, search for related articles, plan your audit thoroughly, schedule a kick-off to check what the customer expects. Basically you'd want to review the hardware, operating system and installed firewall software and all related configuration, installed version, hotfixes, log files, best practices, licenses, contracts, updates, almost everything and create a proper documentation of your results. Check Point provides many tools you can use in addition to your own preparations, CheckMe, CPview, CPSizeMe, SmartEvent, Compliance Blade and so on. Maybe you'd want to conduct a Security Checkup as accompanying service as well (localizations available).