Hard... You're really more likely to get the correct scale by trying to contact the microscope owners, or someone with the same exact microscope setup.
If you're lucky, depending on the software used to generate the TIFF, some information might be stored within metadata. Try uploading the image on http://fotoforensics.com/
and see if you get any relevant metadata. Then you'd have to figure out how to make a scale out of that informaton.