How do I Test a Power Supply

[GermanBeer]

How exactly would I test my power supply if I think it is faulty without having to invest a lot of money on test equipment. How many things can cause a power supply to go wrong to begin with? Can anyone enlighten me on a process that I can use or point me to a good website with such info. this is just a general question that popped into my head and thought it might be good info to have saved for future reference.
thanks.

[galapogos]

Get a cheap DMM and measure your voltages when your system is idle and when it's running something CPU/GPU intensive(or whenever it is you're experiencing problems with it).

[GermanBeer]

Pardon the stupid question but what is a DMM?

[jonnyGURU]

Digital Multi Meter.