1. *Charge the battery with the UPS or whatever. *Attach a 12V tail
lamp. *Time how long it takes before the bulb goes dim. *Compare with
a known good battery. *Not every analytical, but good enough.

Very careful not to reverse charge a cell. Do not discarge further
than 10V. If you are watching the voltage when the first cell becomes
completely discharged you can see a quick voltage drop by 2V,
continuing the discharge would ruin the battery.