The recent coolant conversation shook loose this question, I have a green (happy St. Pattys day) coolant gage indicator thing that shows freeze temp/boil temp. Would it work on Mercedes coolant?

Refractometer versus Hydrometer
 

Jim, if your green coolant indicator gage thingy contains several "little balls," that is a hydrometer. Hydrometers are designed for a specific coolant chemistry. They would not be truly accurate when measuring the freeze point of a different chemistry.
For any coolant, your best measurement tool is a refractometer. Better cooling system service shops keep one in their tool box.
Cheers,