Playing specific games isn't really a sure way to tell. As some games put different demands on gpus than others. Meaning you can play hours on end with one game and experience no problems while another will have problems. Stress tests are a much better, surer, and quicker way to determine if a gpu is wonky. If it doesn't glitch in furmark then it is likely fine. You don't even have to run it that long, I've never seen a faulty card survive more than a minute.
I do both games and benchmarks. Benchmarks can tell you if the performance is within spec, playing the games is akin to quality control where artifacts will stand out more as they detract the experience.
With the [h] reviews and previews, you can see where your system lines up fairly quickly.