What is the best way of monitoring GPU temperature in Xubuntu 14.04 when using the OpenSource driver?

I use xfce4-sensors-plugin:

It needs lm-sensors installed and configured, and it is in the standard (universe) repository.

The ATI sensor (when used with the radeon open source driver) should be called something like radeon-pci-0100 --- Notice however that with recent kernels the card is kept off when not used, so the sensor is not read in that conditions --- see https://askubuntu.com/a/469439/16395

You can check if the sensors are working activating the offloading as explained in this post: http://xpressrazor.wordpress.com/2013/10/08/enable-and-use-open-source-radeon-drivers-in-a-muxless-hybrid-graphics-intelamd-setup/:

1. List the graphic providers:

   xrandr --listproviders
   

   make a note of the id hexadecimal codes

2. Activate them:

   xrandr --setprovideroffloadsink 0x53 0x79 
   

   (you have to substitute the codes of the Intel and ATI providers, in that order)

3. Open a terminal window and run

   DRI_PRIME=1 glxgears -info 
   

Now using sensors in another window you should see the temperature reading of the radeon card going up.