This should be printed on the motherboard, or you can get it through software with CPU-Z (this info should be in the Mainboard tab).
Once you know this, you can use the information in the LGA 771 to 775 motherboard compatibility guide to figure out what similar LGA 771 Xeons are compatible with your motherboard. This guide will only help you figure out what LGA 775 processors are compatible with your motherboard. Note for those installing LGA 771 Xeons into an LGA 775 motherboard In this guide, we'll go over a few methods for determining if your motherboard supports a specific processor.