Your motherboard and components should have temperature sensors on them. There are programs that read those sensors and show you on a graphical interface, right there on your screen. I have a fan controller program that I use for my graphics card. It's called Sapphire Trixx, and it tells the temp. I think CPUID tells the temp of you're CPU. I have an LCD readout on my motherboard for my CPU temp. I am running at @23 celcius at rest, and 38-42 under load.
I wanted to speak about the POWER SUPPLY. It draws dust into it constantly. Overheating of the PS due to dust is a major killer. Clean it out thoroughly and often. And mount it low in the case, with access to fresh, cool air.