PC won't boot, what steps to take?

[JohnnyG]

My mates PC won't boot so I said I'd have a look at it for him. He says the PSU powers up but there's no signal output etc.
His hdd is fine so I'm thinking cpu, memory, gpu or motherboard but what's the best way for me to find out what's gone?
I've got some memory I can try, but I haven't got a compatible cpu

 

[JohnnyG]

I'm not sure what psu is in there as he's not bringing it round 'til Thursday, just wanted to be ahead of things
He says the psu fan powers up & there's a light on the front (power led?) so I don't think that's the problem.
I've got ram, memory & a gpu I can try, but is there any way of finding out if it's the cpu or motherboard if you haven't got a another one to try?