How do I tell if my ground loop has Methanol or Proplylene Glycol as the antifreeze?  Which is better?  Is it OK to mix them?

Not sure how you can determine which you have.  Any chance of asking the installer? 

You can not mix them!!!!

 Methanol water has a low frictional pressure drop and a relatively high heat transfer coefficient. However, it is highly volatile, highly flammable, and highly toxic until diluted.  Low cost.

Propylene glycol solution is an alternative with the only true advantage of being less toxic than  methanol. Below about a 20°F freezing point, the viscosity gets excessively high, resulting in high pumping costs.  High cost.

Thanks for the quick response!

The geothermal was installed 15 years ago when the house was built.  I just bought the house.  I don't know who the installer was nor did the previous owner.

I guess I'll have to figure out how to do a specific gravity test.

Yes, freeze protection is the issue.  To accomodate some remodelling, the interior piping was moved.  This resulted in some fluid loss that we replaced with straight water.