Try cold first!
If it's like mine, it 'clips' to the front grill below the bumper. Remove the clips and then use ice cubes in a cup (with water). That should test the low temperature portion of the thermometer ok.
If it's close there, then you probably will need to get another data point, so hot water would be appropriate. Not sure that MB go to 212 deg F (boiling water). So maybe just hot water measured with a cooking thermometer would suffice for the second data point.
Regards . . . . JimF
'94 S500 Cpe
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