I just type "sensors" at the command prompt to check it occasionally., which relies on the lm-sensors package being installed. If one wants real time monitoring of CPU/GPU/HDD/mainboard temps I would run gkrellm.
Ran a test of my G74 will all 8 threads yesterday being used in Linux. Ambient temp=26C, CPU temps never exceeded 70C.
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G74SX-CST1-CBIL, i7 2630QM 2GHz
32GB DDR3 RAM @1333MHz
GTX560M 3GB DDR5 (192 bit)
17.3" LED 1920x1080
Sentelic TP, BIOS 203
Debian Linux Wheezy (Testing) Kernel 3.2, NVIDIA 295.40