How can you tell if your Video card isn't getting enough power?

[atiVidia]

harrycarry: artifacts are also an effect of memory corruption or GPU algorithm corruption due to excess heat. if the excess heat lingers, the data/algorithm corruption could become permanent i.e. memory failure/GPU transistor burn. artifacts generally arent a problem with cards which dont get enough power, especially nvidia cards. nvidia cards simply slash core/memory clocks in half if it doesnt receive enough power, such as when a molex is unplugged.

[Duffman]

If you don't know what an artifact is, when it happens, you'll see it and go, shit, im a moron. I never saw an artifact til last week when I let ATI tool try to find the max mem on my card while I went to school. I came home 3 hours later, and sat down and wen't shit, im a moron.