If a motherboard has particular functions, you can test motherboard without a CPU. Some motherboards, for instance, offer an onboard diagnostic capability that enables you to check for fundamental issues like memory and motherboard power. It would be possible to test a motherboard without a CPU if you had access to this capability. However, if you don't have this feature, you can't test a motherboard without a CPU. In order to determine whether your motherboard is defective, you must first install a CPU. You can test the BIOS, which will show up on the screen, and perform simple tasks like testing the power supply and memory. If your motherboard won't let you access the BIOS and has no diagnostic tool for checking fundamental operations, something might be wrong. However, if the board has a CPU, it will power on and enable you to access the BIOS or mobo diagnostic features.