Hi Ivana
Wouldn't you use the NWBC in the GRC system to check your full listing of FF Assignments. You can, however, use the local system to update expired accounts (replicates back to GRC)
The IMG activity "Maintain Validity Dates for Firefighter Assignments (Plug-In)" as you both have observed is only for expired users
If you want to avoid the checking in GRC system you would need to look at table /GRCPI/GRIAFFUSR. Otherwise, as Nandita mentioned, you review the user setup from NWBC in GRC.