Resurrecting a 12V SLA from a UPS
Does it have a bunch of specs printed on the side? Check for a manufacture date. If it's more than 3 or 4 years old, it may not be such a prize. Smaller ones (e.g. 1.5A-Hr) can become completely unable to take a charge in that amount of time. On the other hand I've seen 7A-Hr ones last more than 5 years.

I'd put it on a 12V trickle charger for a couple of days, then once the open-circuit voltage (off the charger) is 12.5 or 12.6V, put a load on it and see if the voltage sags. Use something that pulls an amp or so, and if the voltage sags more than a couple of tenths, it means the internal resistance has gone up, and the battery is just plain old.