A quality UPS (not one of those cheap Chinese POS units)
will regularly exercise the batteries, running discharge/charge
cycles and measuring the results. Then the UPS will tell you
the condition of the batteries.

Compared to other types of cells, lead-acid cells are relatively "fragile".
(I once destroyed a $45 Sony battery pack by accidentally letting it run
down.) The /last/ thing you want to be doing with a lead-acid battery is
charging and discharging it to determine its capacity.

And what, pray tell, is going to happen if the AC goes out when the battery
is at the bottom end of the discharge-charge cycle? This is not unlike
punching a hole in life boat to test how quickly it will sink!