One hopes not to go through what phil did... That not only sucks balls but can cause conflagration of mining premises which is never good.
Use your eyes, ears, hands, and especially nose to know! For instance, I knew something was amiss when I could smell that distinctive burnt electrical smell (usuall IC's or electrolytic capacitors) a couple days before my LEPA power supply died. Also, check your PCIe connectors for excessive heat. Cheap or faulty connectors are the scourge and will make a high resistance connection which can overheat.
Just consider the power levels being pushed through these rather small volumes.