1. Check the lm-sensors project site and see if your hardware is supported.

http://www.lm-sensors.org/

2. If so, install configure lm_sensors.

3. For visual display I use gkrellm but you can also use gdesklets (Gnome,
although the gdesklets project has little activity lately). For KDE there
is ksensors

4. If if your hardware is not supported by lm_sensors, your CPU temperature
may show up in gkrellm. This is the case with my Dell laptop that has a
Intel Mobile CPU.