Testing a power supply is quite difficult, the small testers are merely voltage meters and can't tell how the PSU performs under stress, nor can they tell about ripple and other anomalies. They can be useful, though, if the PSU is severely failing. Testers that are able to tell the quality of the power output include an oscilloscope plus other hi-tech gear. Symptoms like sound problems or random blue screens are often the first signs of failure.
As for a new power supply, rather than getting a powerful one by numbers, you should pay for reliability. At the moment it seems that Seasonic makes the best supplies, available also under other brands. You should get enough power, but not too much. A huge amount of extra power will cause stability issues sooner or later. Let's say, some 20% more than your video card manufacturer recommends should be enough.
If you end up getting a new PSU, try to find reviews of the one you're thinking about. The best ones I've recently read include reverse engineering the unit to find out the quality of the components used.
