Below you can see the new case fan that I installed to replace the one that prompted this article. The old fan had failed and wasn't spinning, and the computer was randomly powering off.
Signs Of Overheating
There are other signs of overheating for electronic devices, but these are the most common in my experience.
1. Your computer or device randomly powers off or reboots. This effect is worse on a warmer day.
2. Your computer or device is unstable with the applications you normally run.
3. You hear more noise than usual from the device's fans.
4. You feel more airflow from your device and/or the airflow is warmer than usual.
Causes Of Overheating
The most common causes I have seen for overheating computers (and electronic devices in general) are:
1. Failed Fan: It could be caked with dust, or the bearings could be worn out, or who knows. If it's not spinning, it's not moving air, and whatever part of your computer it's attached to isn't being cooled properly.
2. Dust in Heatsink: The fan has to be blowing air over the heatsink to be cooling whatever its attached to. Anything that does its cooling with a fan will eventually be caked in dust, and need to be blown out with a can of air.
3. Improper Air Flow: If the computer or device is sitting in a corner where its fans can't move much air, it's the same effect as the above two problems. Ideally you want your computer in as wide open of a space as you can get it. And if it must be in a confined space, move it back from the wall.
4. Ambient Conditions: Fans and heatsinks use something called "physics" to cool your device's silicon. If their is too much moisture in the air and/or it's too hot, your device can still overheat, even when it is properly functioning. Not much you can do about that except keep indoors with your AC on ... or move.