tjoyce, I am going to assume you are talking about DB2 z/OS.

If you don't know the name of the SDSNLOAD and SDSNEXIT PDS, you will need to get them from the people who installed DB2. The full PDS names might also by in some of your DB2 JCL in the //JOBLIB or //STEPLIB.

The SDNSLOAD PDS contains the executables for DB2. If you need to modify any of these members to customize your environment (or add your own EXIT routines), you would put these modifications in the SDSNEXIT PDS (and make sure it is concatenated in Joblib / Steplib statements before the SDSDNLOAD PDS).

As for how you can prove they haven't been modified, I don't know how to do that. At our site, both of the members you mention (DSN3@ATH and DSN3@SGN) are in our SDSNEXIT PDS. I don't know why they needed to be modified (I am not the System DBA) but they were modified (or they wouldn't be in the SDSNEXIT PDS).