I will add quite a bit more photos involving the procedure with some hands on guide work later but at the moment i will provide some links and explanations.
1. WHY - After reading the title you might ask your self why you would need to test your power supply. Well for starters they do get old and like any other aging component they do not run as well later in life. Why this could be a huge issue for you is that in most gaming computers we ask a lot of our hardware and if that hardware gets starved of power for any reason even for an instant it could damage other components or just plain cause crashes.
Think about it, when your video card jumps into over time because you generated the coolest explosion ever in your favorite game then just before you get to see it finish the game freezes, pc locks up, you get a blue screen etc. all of these things "Could" be because a power supply did not do its job at that one moment.
2. HOW - Well back in the day it would take some decent electronics knowledge and some pricey multi meter to properly test a power supply. Now and days though we have other options. These options are purpose built power supply testing systems that test all of the different voltages your supply can put out and let you know with a bit of reading weather or not they are within the right tolerances.
These will cost between $5 and $50 depending on what you get but bear in mind that you are protecting upwards of $500-$2,000 worth of computer hardware. So consider it an investment that when you are having a power supply issue is capable of answering the question "do i pay for the rush shipping from new egg or wait a few days and hope the problem goes away before this super awesome game i want to play comes out".
COOLMAX PS-228 Network/ PC Service Tools
I am going to start with this coolmax because it is the one i own that i will use for demonstrations later in this thread. Through newegg it has a price tag ranging from $19.99 (On sale black firday) to currently nearly $40.00. It is a solid VERY simple build that has yet to fail me. You simply plug in your mobo 20 or 24 pin cable bundle to one end and the cpu 4-8 pin on the other and it will give you a read out of the exact voltages at each pin and sound an alarm if there is any irregularity at all. Considering almost all parts are built to the same standards (Maybe not always the quality but the power expectations) this thing will also test your Molex connectors and SATA power connections as well as just about any other pin out from the Power supply that you might want to test.
It is a great little tool for the price and it keeps you from guessing. Before i bought this i would try things like plugging in less hardware unhooking my video card playing with different configs wasting up to an hour or more just messing around. Now i just unplug the PSU from the PC plug this in (Without having to take the cables out of the case mind you) and just test it in place. Within a minute my answers are in my hand on a rather nice blue LED screen.
Here is another version with high ratings. I do not own it but i might pick it up since i loan out my above tester so frequently.
Thermaltake AC0015 Dr. Power II Power Tester