A digital multimeter will do the job.
There's no way of properly checking the voltage of the battery while the PC
is on. You have to remove the battery to isolate it.
The is no utility to check the voltage of the battery when the battery is
still in the PC and XP is running. Taking the battery out to test it with a
battery tester / multi meter will cause the CMOS to erase itself, back to
defaults.
One quick check is to make sure that your PC is not loosing date/time
settings. If it is, the you should change the battery, after noting down the
setting in the CMOS on paper.