delaro :
Nvidia cards can't have the firmware flashed, so the only thing you need to look for are do the fans work and how is the heat underload. You will find that hard core gamer are harder on cards than miners since miners underclock and try to find the best performance/power ratio vs overclocking.
PassMark or 3Dmark for benching
https://www.3dmark.com/
https://www.videocardbenchmark.net/
HWmonitor for heat and voltage
https://www.cpuid.com/softwares/hwmonitor.html
He can also use GPU-z to determine if the cards bios displays correctly and all things should be as they are.
Don't necessarily agree with hardcore gamers v miners observation though. Id much prefer a card that's used for gaming maybe 6 hours a day for a year, over one used 24/7 by miners. Just my opinion though.
To the OP, if you can get the card to test, it seems like you know the seller. Is it someone you trust? can you tell/ask if it's been used for mining or general gaming use?