Install lm-sensors package:

$ sudo apt-get install lm-sensors

Execute sensors-detect command as root:

$ sudo sensors-detect

It will determine needed modules and at the end it will ask for your permission to automatically add them to the /etc/modprobe file. If you answered yes then load them using command:

$ sudo service module-init-tools start

Now you can use sensors command:

$ sensors
Adapter: Virtual device
temp1:        +44.0°C  (crit = +100.0°C)
temp2:        +44.0°C  (crit = +100.0°C)
Adapter: ISA adapter
Physical id 0:  +45.0°C  (high = +86.0°C, crit = +100.0°C)
Core 0:         +45.0°C  (high = +86.0°C, crit = +100.0°C)
Core 1:         +44.0°C  (high = +86.0°C, crit = +100.0°C)