To test your UPS, throw the circuit breaker with the UPS on it to simulate and outage and see how the transition goes. Note that in general testing an UPS by pulling the plug from the wall is not a good idea. Electronics like to always have a good ground reference. If you unplug a UPS, it's still powered but now has what electricians call a "floating ground". Not only can this be bad for electronics, but it can be quite dangerous as well. It is likely that unplugging just about any UPS for a short amount of time isn't likely to result in disaster (don't take our word for it, though!), but in all cases, throwing a circuit breaker would be a better thing to do.
It might be useful to install a GFI (Ground Fault Interrupter) on your UPS-covered outlets to facilitate this testing without having to throw a breaker, especially if you don't have your UPS protected machines on an isolated circuit (which you probably should). These are the sockets found in most modern kitchens and bathrooms with a red and a black button. You push the latter to cut power and the former to restore power.