You have to first decide on your scale of interest. At small scales (micrometer to nanometer) the usual descriptor is roughness not flatness. If surface roughness and its measurement and quantification is a new area for you, the following Wikipedia article may be of interest: https://en.wikipedia.org/wiki/Surface_roughness 

Someone at your institution may have equipment to quantify the surface roughness of your "fisherfinest" glass slides. 

Is you describe why you need the flattness probably you will get more answers. Do you need it to pprepare homemade cuvette or flow chamber of for other purposes, where the equal distance is main goal. Some colleagues were making liquid crystal cells for some kind of testing long ago using similar cover glasses

Slide flatness is very important parameter in automated microscopy but slide manufactures do not give the information. In light microscopy with focal depth up to 100 nm glass flatness can be measured using the microscope stages with known planar characteristics. Usually the parameter is more than 50 arc sec