We test our power supplies under artificial load conditions and using in-house calculated load levels to check that a PSU does what it says on the tin.
While the industry is clearly geared to "real world" testing scenarios, in this situation anything we throw at it should be at most equally stressful in the real world.
The recorded results on the test machine are also manually verified with a multimeter to check there is no voltage drop between the PSU connector and the load machine to ensure accuracy.
80 Plus Program
The 80 plus program is an independent certification that PSU manufacturers are now going after in order to fully ratify their products as capable of 80+ percent efficiency at 20, 50 and 100 percent loads and PFC of over 90 percent. It's an electric utility-funded incentive program to force companies to make more efficient products as wattages spiral ever-upwards, in order to save the consumer as much money as possible.
Traditionally people have looked for the Energy Star rating, of which 90 percent of computers on the market currently adhere to, but this was last revised in July 2000, which is somewhere in the region of a millennia in computer years. An updated Energy Star specification (version 4.0) came into effect from July 20th, 2007 and includes the 80 Plus program's requirements.
In this way even a small increase in efficiency can save quite a considerable amount of money (and the environment) in the long term.