I've said it before countless times, but "mined on" GPUs are exactly identical to ones that have been gamed on. There is no detectable difference - you have literally no way of knowing. The reason you have no way of knowing is because mining doesn't do anything different than any other use of a graphics card. The only thing that *might* matter is that fan bearings tend to wear out much faster (in terms of years of service) because they're running essentially 24/7. Everything else in the graphics card is exactly the same as new, same reliability - actually higher, since failure rates on new cards are highest right after purchase due to them not having been tested for sustained (days to weeks) periods.
I've bought cards that had clearly been mined on in a shed and were filthy. A few with actual water damage or rust on the coolers. You know the worst thing that happened? I had to replace some fans. I had multiple RX 480/580s that were mined on literally almost constantly for 7+ years and are now in service in other PCs. All of them still work.
This whole "mined GPUs are risky" thing is bitterness from people who didn't mine. Oops, I said it. It was annoying for me too, which is why I started mining. I would much, MUCH rather buy a used card from another miner than from a gamer. Miners were overwhelmingly doing it for profit, which means you need to keep your equipment in good working order. The average gamer doesn't even begin to know or care about PC maintenance.
Mining GPUs are fine. Don't buy them from China or overseas, obviously, but if you're in the US, Canada, western Europe, etc then the only thing you have to worry about is *possibly* replacing fans. Most cards that aren't from Zotac don't even require that much. Zotac cards pretty much guarantee the need for fan replacement, so keep that in mind. If replacing fans or thermal paste scares you, guess what: your only method of being certain a GPU hasn't been mined on is if you buy new. There is no way to tell, there is no physical evidence, and nothing stopping anyone from lying to you and saying "oh yeah for sure this card was never mined on, totally."