Laylow;
These type of UPS's are pretty good, the ones that just flat don't come on even plugged into the wall means that the board on the top has been shorted out and the unit is only good for spare parts.
However if you get units that do come on, then one thing you could do is plug it into an AC and let it set "ON" for about 30 mins; then check the battery. It should read way up close to 13Vdc out of circuit. This will means the battery is still good.
Anything else and it's toast.
The reason for this is that APC's smaller UPS's are really bad about constantly over charging the battery and over less than 6 months (which BTW is just long enough to have the battery out of warranty). This happens to some of the older 1400 and 3000w units as well and unless you are dilligent about up-time load testing you won't find out until you really need the unit to work. With this in mind, you could also take the batteries out of the units one at a time and shake them listening for "rattles", if you hear ANY, the battery is toast.
Usually for a small fee, you can take the batteries in question to places like batteries plus and ask them to do what is called a "Lifecycle" on the SLAs this will take a few hours to test, but it is the end all of tests. Not only will it tell you if the battery is any good , but also tell you how much of the original load size is left and will also try to regain some back by "cycling" the batteries. The old small NiCds that I've sent to a few here on the board, go through this very test wit the very same type of tester that these places have.
If you get them for free take all of them, if they cost try offering now more than $3 - $5 /ea of even say $50 for the lot. As you really don't know what shape the batteries are in.
BTW: I have a smaller 200VA and it's what I run my freezer off and it's doing just fine.
As some one else said if you can get the schematics...good. There is a way to bypass the "look for AC first" logic and will be able to use these without plugging into the mains, but you'll also want to find the logic for the beeper as will will be on too.
Hope this helps some
Bruce S