Our standard checklist and troubleshooting thread:http://www.tomshardware.com/forum/261145-31-read-posting-boot-problems
To do a basic check out of the PSU:
Try to borrow a known good PSU. (Brand new, out of the box, untested PSU is not a known good one.) If you cannot do that, borrow a DMM to measure the voltages. Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. Tolerances are +/- 5% except for the -12 volts which is +/- 10%.
The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.
You can turn on the PSU by completely disconnecting the PSU and using a paperclip or jumper wire to short the green wire to one of the neighboring black wires.http://www.youtube.com/watch?v=5FWXgQSokF4&feature=youtube_gdata
This checks the PSU under no load conditions, so it is not completely reliable. But if it can not pass this, it is dead. Then repeat the checks with the PSU plugged into the computer to put a load on the PSU.