In my lab I have a ATX Power Supply Tester.

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If you want to buy, see here:

Vendor	Product
eBay USA	Plastic Digital LCD Display Power Supply Tester For 20/24 pin ATX SATA PCIE 6pin

Documentation

I keep my documentation for this equipment here:

100% Brand New and High Quailty
This product is designed to show PC ATX power voltage on LCD display.
Easy to use - Simple tester for switching power supply.
Aluminum Shell , Good for radiating.
For ATX PSU each output connectors check.
Support Tests:
- 20-pin and 24-pin power supplies.
- For SATA power.
- Pentium 4 power connector.
- PCI-Express power connectors.
- For Xeon power connectors.
- For Floppy drive connectors.
- Standard 4-pin power supply connectors.
Accurate voltage indicator +/-V(+12V1/+5V/+3.3V/5VSB/+12V2/-12V).
For ATX P.G value display.
Lower or higher P.G values alarm.
Lower voltage detected alarm.
Over voltage alarm.
No voltage detected alarm.
Size :(Approx.) 12 x 6 x 2 cm
Voltage source : 20/24 pin ( ATX - connector )
Voltage test : +12V , -12V , +5V , -5V , +3.3V , +5V
Stand by (SB) 12V power Good ( PG )
Connectors : Floppy, HDD , CDROM , SATA , 4pin ( P4 ) , 8pin ( Dual-CPU ) , 6pin ( PCI-Express )
Color : Black