To be honest, in all the years I have been building systems, I have only had 1 go out, and that system was like 8 years old. They are the same battery that most watches or keyfobs run on. They are supposed to last 5 years or so.
Easiest way to check, is to use a DVOM like Latios said, or make sure all the settings and date are current in the BIOS, and shut down the system, then remove the power cord from the PSU. Let it sit like 15 minutes and power it back up. If the settings and date stay the same, the battery is good, if not then the battery is bad.