Your computer's motherboard is like a central nervous system, connecting your computer's different parts together and coordinating their activities. If you ever need to replace your motherboard, it's important to know which type you have.
Look at the start-up screen of your computer when it first boots. You may be able to find information about your motherboard's manufacturer, along with its model number. This information will typically be displayed after the initial start-up splash screen, and will only be present for a second or two as the computer goes through boot checks. When this information appears, hit the "Pause" button on your keyboard to freeze the screen--so that you can take note of what's displayed (press any key to continue the boot cycle when you're finished).
Look at the motherboard itself for the name and model number. Turn off and unplug the computer, remove the side of the computer case and examine the motherboard closely (the motherboard will be the largest circuit board in your computer with lots of other parts connected to it). The manufacturer's name and the model number may be written on the edge of the board or located on a sticker in plain view. You may have to remove accessories from the board in order to get a clear enough view to examine it fully.
Call the manufacturer of your computer and ask them what type of motherboard it has. This requires some patience and work to perform, but it is a way to identify your motherboard with absolute certainty.