Upgrading issue with customized code in D365FO from PU10 to lastest PU version.

 Hi. I'm trying to upgrade version of D365FO from PU10 to lastest version.(On-premise)

 One of my customers is demanding to upgrade their D365FO version, so I need to check system integrity.

 Problem is how I can check their developed codes when I upgrade version of D365FO.
Because there are several developed modules in the system, like VAT module, Human resources module, etc.
(They are divided in each specific models)

Please let me know the way to check code integrity in new PU version.