When it comes to testing a power supply, there are two courses to travel. One takes you down a path using a device to stress out the PSU and provide data regarding the power levels across all three rails. The second, and the one I make use of, utilizes an actual test system to give a more real-world account of what the power supply is capable of. While both methods have their merits, I prefer to use an actual computer to more closely resemble the manner of use that you, the potential customer, will put the product through.
That said, let's take a quick look at the test system. At the request of readers, I have beefed up the system to put a more "power-hungry" level of strain on the power supply.
Gigabyte 965P-DS4 motherboard (Supplied by Gigabyte)
Intel Core 2 Duo E6600 processor
2x 1024MB Corsair XMS2-8500-C5 memory (Supplied by Corsair)
GeCube X1900XTX graphics (Supplied by GeCube)
Sapphire X1900XT graphics (Supplied by Sapphire)
Western Digital 250GB SATA hard drive
2x Western Digital 160GB SATA hard drives
Western Digital 80GB hard drive
Sony 52x CD-ROM optical drive
Samsung 16x DVD-R optical drive
2x 120mm fans
4x 80mm fans
Testing will consist of checking the power levels across all three rails at idle and again while the system is under stress. This should give us a good look at the capabilities of the power supply being tested.
When it comes to looking at the voltage results, I always pay special attention to the differences between idle and load. While good idle levels are a good starting point, a PSU that cannot continue to provide those high levels under stress won't be very beneficial to us.
The GPX750 is an example of what we like to see when testing. All voltage rails show readings slightly above the stated rail specifications while under idle conditions and remained that way even when straining the test system. Even when stressing the machine with a pair of very hungry video boards doesn't phase this power supply. This goes far in giving a good picture of what you can expect when using an enthusiast level computer.