Taken straight from the PSU faq (and written by me as well

)
QUOTE
Q: How do you measure voltages?
A: With a digital multimeter. Preferably at the main ATX connector, but molex connectors/AUX connector are also fine as well. The yellow wires are the 12v line, red is 5v, orange is 3.3v, and black is ground. Software monitoring programs like MBM5 or BIOS readings can vary by accuracy and should be taken with a grain of salt.