CA Librarian does not have a central control file or database that identifies the Master files.
Hopefully, some sort of name convention has been used and incorporated MAST into the data set names.
You can run a Master Index report like this to verify if a data set is a Librarian Master file or not:
//INDEX EXEC PGM=AFOLIBR,PARM='NRJS,NJTA'
//SYSPRINT DD SYSOUT=*
//INDEX DD SYSOUT=*
//MASTER DD DISP=SHR,DSN=dataset.to.check
//OSJOB DD DUMMY
//SYSIN DD *