An UPS is an Uninterrupted Power Supply. It contains a battery that is used when your power goes out or there is a brown-out type condition where you are not receiving full power. It will also stop surges and spikes where there is a momentary over amount of electricity or shortage of electricity.
The UPS makes sure your machine and devices keep running thru these interruptions.
You do not want to plug a laser printer in a protected port on the UPS as the laser printer regularly warms up and pulls a bunch or power and would quickly drain the battery. Besides in a rough electric time do you need to print? More than likely the things you want running are the computer, monitor and external disk drives.
Testing a UPS involves stopping the power. Only do the following items when people are not using or accessing the computer which usually means after work hours in the evening or nights or weekends.
On some UPSs there is a button you can push, and it will show you an indication if it is working. On others when the battery gets weak it will start beeping. However, it may also beep when the UPS is supplying power. When I was in Brazil in 1999 working on computers in the evening, they had a UPS on each PC and the power was lousy in the evening and the UPSs were continuously beeping showing they were supplying power as it dropped
To find out if your UPS does work the best thing to do is pull the power cord to the UPS from the outlet and see what happens. If your computer (not a laptop, they have a battery built in which is a UPS on them) stops, your UPS is bad. If the computer keeps working, then the UPS is good. Keep in mind most UPSs only will keep equipment running 5-15 minutes. You could time as you do this.
UPSs do wear out and need replacing just as other batteries you use.
Thanks, Chip, for the question.