Dead Power Supply Unit (PSU)? How to be sure ?

So you turn on the computer and nothing happens, it could be a broken case switch and therefore the PSU never gets the signal to turn on. Or it could be a dead PSU or something else dead pulling down the output of the PSU.

 Here's what you do:

1) Unplug the leads to the case switch at the mobo and jumper them with a small piece of wire. Now turn on the main power switch at the back of the PSU. If nothing happens, we're a long way to proving the PSU is dead.

2) If above procedure caused nothing to happen, now we get a little more into it. Remove the 20pin PSU connector and the 4pin CPU connector to the mobo, all other psu connections can stay put. Using a small wire (a paper clip will do) jumper pins 14(the only green wire) and any BLACK wire (let's say pin 13 or 15) on the 20 pin PSU connector. Now turn on the main PSU switch. Your PSU should come to life (HDD spins up, and fans spin).

Below is a pic of the 20 pin connector 

From the pic you can see that pins 13 and 15 are ground pins, that's why you can use either one. You want to ground pin 14.
This is all the case switch does via mobo traces out to the case switch leads. Well it does go thru a FET or Bipolar transistor to make the actual connection, this is how Windows can soft shut down the PC.

3) If nothing still happens then start removing 4 pin molexs one at a time, starting with the video card (if it has one). This will eliminate any peripherals "holding" or "pulling" down the PSU. After all connections are removed and still nothing, then your PSU is dead for sure.

4) Edit: I forgot this possibility. If after step 2, all fans spin up and HDD spins, but at step 1 nothing works, then it's most likely a faulty mobo. And most likely that little FET or Bipolar transistor or at least the circuit that turns that on. Either way at this point you're best to take it in for service or RMA your mobo back if applicable.

Hope this helps