Click the Apple-shaped logo in the top-left corner of the screen. A drop-down menu will appear. It's near the top of the drop-down menu. Look at the number to the right of the "Serial Number" heading. Open a search engine of your choice (e.g., Google), then type in your Mac's serial number followed by the word "motherboard" and press ⏎ Return. This will bring up a list of matching motherboard models.

Summary:
Open the Apple menu . Click About This Mac. Note the serial number. Look up your Mac's motherboard model.