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.
![Tongue](https://bitcointalk.org/Smileys/default/tongue.gif)
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.
![Wink](https://bitcointalk.org/Smileys/default/wink.gif)