The problem with darkroom thermometers is that they are designed for darkroom temps so don't measure either water freezing or boiling point but cover the much smaller and useable range appropriate to a darkroom. So the tests described above aren't applicable.

I can't think of any easy way except to buy or borrow a certified thermometer which is guaranteed accurate to within plus or minus 0.2 degrees C

The differences mentioned are large and certainly significant for both B&W and especially colour.