Unplug everything from the motherboard except the CPU with its heatsink and fan, power supply, power switch and case speaker. Switch on and you should get a BIOS beep indicating that there is a problem (no memory or monitor/graphics card). This won't be the short single beep that indicates that everything is OK, but a long beep or series of beeps indicating a problem. If you do get a beep signal, refit the components starting with the monitor and memory until you fail to get the beep. You will then know what is causing the failure.
If you get no beeps the mobo is suspect.