Is this a laptop or standard desktop/tower PC?
If it is a laptop, just unplug the thing, and if it still works, then your battery works..
If it is a desktop, then turn it off and unplug the power cable for about 30 seconds to a minute. Plug it back in and turn it back on. If your computer still has the correct BIOS settings for the hard drives, CPU, memory, etc. and the system clock still shows the correct time, then your CMOS battery is working properly. If these settings are not correct, then you need to replace the battery..