The BIOS reports it, but it relies on sensors on the motherboard, which could be wrong. When using the volt/ohm meter, monitor the 12v rail, as that is one of the more important, and most used ones, then monitor the 5v and 3.3, if you don't drop much, or gain to much, then your PSU should be fine, but if you get huge drops/gains in voltage, or it fluctuates drastically, and quickly, then the PSU is a bad egg.
BTW, all yellow goes to the same place, same with the other colors, unless you have multiple 12v rails, then you will need to test a 12v cable from each 12v rail, which can become difficult in finding a cable on a different 12v rail.