There's a colour code for these:

Yellow = +12v
Red = +5v
Orange = +3.3v
Black = Ground

Other wires are:
Green, Grey, Purple and sometimes white.

However, voltage is only part of the story. These voltages need to be
stable over a wide range of current draw and also have minimal ripple
on them. It's possible for a PSU to give all the correct voltages but
fail to operate a motherboard properly, or cause instability due to
poor regulation under load.

If in doubt the acid test is a replacement PSU.

FWIW - on opening up many marginal or failed PSU's I've found the old
'bad capacitor syndrome' and in one spectacular case one of the coils
had failed insulation and had burned black ( customer had reported a
flash and bang from the back of the PC !! )