If you download screenlets (type the name in the deb section of the software portal) there are a couple of widgets you can fix to your desktop that will show you the core/ambient temperatures of your GPU as given by Nvidia settings. I've dotted the ones I've found to have the option around my desktop in the screenshot linked below. Moving from left to right and then down they are:
"Ringsensors" (GPU core, GPU ambient)
"Meter" (GPU core, GPU ambient)
"Sensors" (GPU core)
"Nvidia Screenlet" (GPU core only)http://img156.imageshack.us/img156/2827/screenshotlv1.png
The screenlet on the far right is sysmonitor which is the only screenlet I usually have but it unfortunately doen't have the temperature options.