I just bought a Corsair HX850 and a Asus P6x58D MB. I want to test if my PSU works, but is this possible when there's nothing installed on the motherboard yet. The only thing hooked up right now are some case fans. Could I damage the MB?
You can't test it with just the mobo. The 24-pin connector to the mobo acts as a switch to the PSU. IMO, you'll need to have at least a mobo, cpu, and a case (to turn on the pc) to be able to test if it is working.