Motherboard manufacturers have been using the same CMOS batteries since, well forever. They are all shiney, round about the size of a nickel (models CR2025/CR2032) and very easy to spot on the MB.
However, if you go back early enough, there were variations before the standard became the standard. Some motherboard manufacturers, put their batteries in a covered box mounted on the MB or even as a seperate card. They didn't even all use the round button batteries: any battery was fair game as long as it would hold a charge for at least a year and were convienantly mountable and replacable. I've seen watch batteries, camera batteries and even AAA's.
So, in the end, if it isn't obvious, and I assume by your question that it is not, you'll just have to search the MB for the battery.