Stress testing it would be best and watch the temperature of the GPU through the process to make sure its operating within spec. Have it grind out a heavy GPU processing benchmark for a good 10 minutes or so to really get it to heat up. More info here:
https://www.wepc.com/how-to/stress-test-graphics-card/*You will also want to pay attention to the display as its grinding out the stress test and watch for glitches and artifacts and stuff like that. Additionally if the person selling the video card knows it has a flaw they will probably chicken out the minute you say you want to run a stress test on it as it will catch them trying to pass off a bad/damaged video card and they will likely refuse to allow running it and want you to buy it as-is without the stress test and usually the price tag lower than it should be is an indication that they are trying to dump their problem card.
*Lastly if the video card potentially has a valid warranty left on it, as the video card hasn't been altered by anyone, you could potentially buy a damaged card and submit it for warranty repair/replacement and have a good video card in the end if your willing to deal with the RMA mess of sending a video card out and getting it back. Asking the seller for the serial number info on the video card first, you can look it up to see if it still is covered by a warranty before buying it.
A friend of mine got a dead 3 year old hard drive for free that someone was throwing away and it was still covered by a 5 year warranty so he contacted the hard drive manufacturer and placed a warranty claim and they shipped him a healthy replacement
Only cost to him was the shipping fee to send in the dead drive which was like $8.00 USD, which was a bargain when the drive was worth about $50 at the time healthy new. So he figured he saved $42 by getting a hard drive that retails for $50 for just $8.
