You could try to guess based on dark spots maybe but that's more like gambling. Boards that smelled "burney" worked fine while others just failed - the cause of

You could try to guess based on dark spots maybe but that's more like gambling. Boards that smelled "burney" worked fine while others just failed - the cause of failure was heat as in most cases the thermal pads weren't touching the heatsink or the component, were missing or were crumbling. SMD elements can fail either open or closed/shorted, the latter is more likely to cause a worse failure but it's not exclusive. Due to protection circuits elsewhere they can fail without visible marks so it's hard to diagnose. Thermal pads dry out because of heat first, their quality second, with load coming in last imo. So the fact that it's been used for mining alone doesn't mean much. The conditions under which a card was used and the time matter more - if only sellers told you the average temperature, case type, ambient temperatures, whether the cards were undervolted/overclocked and how long they actually ran...