Open Speedfan and run Prime95 on the "Small FTT" setting.
While running both programs monitor your 12V rail voltage on SpeedFan. If you see a lot of fluctuation like the voltage dropping from 12.09 to 11.88 for example then most likely the psu is not that stable.
If you check reviews from good psu manufacturing companies you will notice good psus only drop like 0.1 (at the worst) between idle and 100% load states. (check this review http://www.pcmoddingmy.com/content.php?review.228.3)
I can tell you I've seen the huge difference between having a crappy psu and an Antec TruePower II for example. No doubt there are a lot of other excellent brand names.
Another good way is to do what The Bartender Paradox
suggested. Get a voltage meter and plug it in a 4 pin molex connector. In fact this is better than using software because it's more accurate. But then again, not all people like to fiddle with voltage meters