If you want to ensure you have the correct voltage reading you can try measuring it with a multimeter.
Take one of the molex connectors and the black wire of the multimeter should go to the black wire of a molex connector and the red one should go with a yellow one.
Picture below is not mine but that's how it should look:
![](https://ip.bitcointalk.org/?u=http%3A%2F%2Foverclock.pl%2Fimages%2Farticles%2Fcontent%2Fmcdracullo%2Fartykuly%2Fmierzenie_napiec_w_zasilaczach_atx%2Fmie_02.jpg&t=663&c=rwGlLZv4ZMALLQ)
Don't forget to measure the voltage both when the rig is idle as well as when it is under full load.
if you're going to do that, make sure your multimeter's margin of error is less than +-0.1 volts. Because anything more and you're just measuring noise. Also, if you're on a mult-rail PSU, make sure you measure each rail.