They put in another card and retest.
I know what you are asking but PC makers have been lax about issuing test or diagnostic software. The reason is simple. Such software is expensive and swapping a card is cheaper.
Your issue can be a failed fan on the graphics card or software but again there is no diagnostic software worthy of mention. You'll find that shops across the globe will clean the heatsinks, replace fans and reload the OS.
My wife is starting to have problems with her pc. The flatscreen goes blank and nothing seems to get it working again without shutting the machine down and restarting.
The screen is about 2/3 years od but the graphics card is older. Is there a test to see if the graphics card is failing (it's a radeon)?