A good way to test whether or not it's the CMOS battery is to apply some obvious change in BIOS (such as change initial numlock on/off, change boot order, enable/disable something). These settings (as well as your system time) are stored in volatile memory (memory that will lose data if it has no power). The small CMOS battery provides this power when there's no main power (such as main battery or power supply), and if it dies and your system has no alternative power source to keep the data, it will lose it- and your system will revert to default settings.
So the theory is, if you make a change in BIOS (something not default), and your clock suddenly gets messed up, if you look back at the BIOS settings and notice that your change in BIOS is also reset to default, it's likely your CMOS battery is kaputt.
If the change in BIOS you made did not reset itself with the clock reset, it's likely that the problem lies not with the CMOS battery, but somewhere else (likely a software issue).
On most laptops, the CMOS battery is user-accessible, and is generally replaced by removing the RAM cover or mini-PCI cover. Some require you to remove the keyboard.