If you think that your computer is overheating, you can check the temperature of the CPU, the hard drive, and of the other components that the sensors included with your hardware measure. It's important to do so because many electronic components can be damaged by excessively high temperatures.
The instructions below will show you a couple of different ways to check your computer temperature.
1. Check the computer temperatures with BIOS. Not all BIOS include the temperature monitoring options, so you might or might not succeed with this step. To get into BIOS - turn the computer on, and while it's booting, look for a line that says something like "Press F2 to Enter Setup", but it could be a different button, like F1 or F10. Press that button to enter BIOS and look for a tab named something like "Hardware Monitors" or "PC Health", and you will find the temperatures there.
Note, however, that while in BIOS, your computer is not doing all that much, so the temperatures might be lower than what you normally experience.
2. Download a software program that can check the temperatures from within the operating system. There are quite a few of them available for Windows and Mac, but with some searching you can find a Linux program as well. I'd recommend Everest (works on Mac too) or SIW - look for links in the resource section.
3. Once you have installed the program, run it, and then check the temperatures under Hardware->Sensors tab in SIW or Computer->Sensor in Everest.
4. Note that normal CPU temperatures run anywhere from 30 to 60 degrees Celsius. If yours is higher than that, you might want to have your computer checked out before it gets damaged any further.