The methodology most used to review power supplies is just adding a multimeter on the power supply outputs and measuring if there are any fluctuations on the voltages found there. Some websites even compare the voltages found with voltage levels found on competing products (some examples here, here, here and here). The problem is, this procedure is wrong and tells us nothing about the power supply.
Reviewers that do reviews like this probably think that at least they can see if there is any fluctuation on the power supply outputs, however in reality they simply won’t be able to measure this.
The idea of measuring a power supply with a multimeter comes from linear power supplies, where the power supply has a separated voltage regulator circuit (normally done by an integrated circuit or by a zener diode, sometimes with the aid of a power transistor). In this kind of power supply it makes sense to use the multimeter to check whether the regulator circuit is working fine or not. Even in this case, simply attaching the multimeter won’t let you know if the power supply is being able to provide its labeled current/power. You will need to add a load to the power supply outputs.
On linear power supplies, as they are an open-loop system (more about this in a moment), the output voltage can increase or decrease according to the applied load – so the idea of attaching a multimeter in parallel with the load to check if there is any voltage fluctuation depending on the load makes sense.
Power supplies used on the PC use switching-mode technology, which works in a very different way. They are closed-loop systems, meaning that the power supply measures its output voltages and corrects them if there is any fluctuation. This is done by the PWM circuit, which is in charge of switching the primary transistors. In other words, if there is any fluctuation on the output voltages, the PWM circuit will know it right away, increasing or decreasing the duty cycle of the signal applied to the switching transistors in order to correct this. Since the frequency of the signal applied to the transistors are in the range of kHz, it would take only a few microseconds to the power supply to correct any fluctuation found on its outputs. And no multimeter would be capable of measuring the power supply fluctuation, if any.
Also, since the power supply found on the PC have five different outputs (+12 V, +5 V, + 5 VSB, +3.3 V and –12 V) you would need to connect five multimeters to the power supply at the same time, and publications using this methodology usually use only one, measuring the outputs in different moments in time, making the results worthless, since they were taken in different moments, with different conditions (load, temperature, etc). Even if you connected five multimeters, you would need to read them at the same time. We don’t know any human being able to read five instruments at the same time and write down the values at the same time. Even if you are really fast, you will take some seconds to make this measurement. As we already mentioned, things inside the PC power supply happens in microseconds, so seconds make a huge difference.
One way to use the above methodology correctly is by using a device to grab the value of all five outputs at the same time, like a digital data collector. The problem is that we would measure the voltages, which, once again, mean nothing. One way to make a correct power supply test using this approach is by measuring the current (and not the voltage) of the five outputs at the same time using a data collector, if you add a correct load to the power supply.
Another problem regarding the use of regular multimeters is precision. We cannot guarantee the precision of low-cost multimeters. If you add five multimeters, we cannot guarantee if the multimeters are calibrated among them, showing the exact same results when measuring the same thing.
In our experience, only very low-end power supplies fail voltage regulation tests. Since manufacturers do not send this kind of power supply for reviewing and most websites don’t buy products to be reviewed on the market (they only review what manufacturers send them), most websites will never see a single power supply failing voltage regulation.