I would rather know about a developing problem than wait until the cabin alt warning goes off. Therefore I check the system periodically during climb, cruise & decent - more often during climb & descent.
During one descent in a B737NG, I was surprised to find the cabin climbing at around 500fpm when I checked the system at around 30,000’. After watching for a short period of time to see if it was a temporary condition, it was obvious we had a problem as the ROC was constant. I directed the F/O to disconnect the A/T & push the power up to around 60% N1. The cabin then went from a 500fpm climb to a 600-700fpm descent. The system stayed in the normal mode, so hadn’t detected a fault. We used speed brake against the power & with a lap of the pattern on the way in (ATC requirement, not ours) we had plenty of track miles to get the cabin down before landing.
The engineers checked it out & found a sticky outflow valve was the issue. Too much foreign material had built up in the mechanism. They couldn’t tell me why the system hadn’t detected the fault & swithed to ALTN. They did concur that it wouldn’t have made any difference anyway, given the nature of the problem.