The corrections in GDDR5 are handled invisibly so for the most part, the only symptom is FPS stops increasing then starts to go down as the clock continues to go up, just as you'd expect. I am not aware of any auto-overclocking program that charts FPS vs clock, so that's something that must be done manually for each card.
Artifacts and crashes don't start to appear until the errors are so numerous and large that they overwhelm the error correction, well beyond the point where performance has degraded.