Is it possible to test an UPS battery without having the UPS nearby?
A simple $20 voltmeter will give a reasonably good indication, especially if the batteries have never been used. A lead acid battery should quiescently show 2.0 volts per cell. If the cells have failed, they readily show less than 1.0 volts per cell. For other battery technologies, other voltages apply.
Looking through my UPSs, it seems some use 12 volt batteries (6 cells in a case) and 24 volts (12 cells). The one which has been sitting unused for over a year is a 12 volt and shows 11.3 volts. That would indicate it is in fair condition, but certainly not new (I recall the charge controller had issues, not the battery).
A simple voltage test is far from conclusive. Rather it is equivalent to taking your temperature to see if you are sick. Temperature doesn't show all cases of sickness, but it is more than useful enough as first diagnostic.
For a more thorough battery test, a suitable battery load tester is useful. While inexpensive load testers are sold for car batteries, those use far too large a load for a UPS battery. Something in the 2 to 10 amp range would be more suitable, unless it is a mainframe-class UPS. You could MacGyver a small load, like a car headlamp in tandem with the voltmeter. Look at the battery voltage, enable the load. The voltage should drop a little, like 5% to 15%. A weak battery's voltage will plunge more than 30%.
Go to ANY motorcycle shop and buy a battery tender and connect them up to your individual 12 volt batteries (remove all interconnects from the battery packs).
The Battery Tender will tell you when it's charging, or when it switches to "trickle" (green light comes on, or blinks green).
If it remains on CHARGE (red light, or on some types a yellow light) after 8 hours connection, the battery is bad.
Generally, on a multiple battery pack in a UPS, it only takes one battery to go south to have the whole bunch report as bad.
When you pull the pack out and connect the Battery Tender, lots of times it will charge for a very short while (often minutes, if not seconds) before the green light comes on. If that happens, good battery. Go to the next one in the pack and do the same.
Remember: Only test one battery at a time.
Remove all connections from the battery terminals before connecting the alligator clips from the Battery Tender.