I need a tool to get the exact temps. Whats my best bet?

As far as the themostat - temp gauge goes up to just below 1/2 mark. I assume this is where the thermostat opens because temp then drops back down to between 1/4 and 1/2 mark and stays there - until recently.

It is also the autozone thermostat. May be going bad and not opening all the way?