They are about €1 for a small pack. So it’s easier to replace it than to test it directly.

But if you mean to see if it’s the problem without taking it out.

After running the computer for a few hours. Turn it off, unplug it from the mains and disconnect the main battery for 24 hours. Connect the main battery and power it on, go directly to the bios and see if the time and date are still correct.

If they are not, it’s likely a bad CMOS battery.