Identify Emulsion Side Of Slides

I've convinced myself to use an Arduino to assist in scanning about 20k 35mm slides I inherited.
Tongue-in-cheek, pretty straight forward to move the slide carrier, raise the slide, check its portrait-landscape orientation optically and if necessary rotate 90 etc, blast some air on it, scan, then replace in carrier, repeat ad-nauseum, probably for a few months.

But some slides are 180 degrees reversed in the film plane, ie the emulsion is in the wrong place for ideal scanning results. Ultimately it may not matter that much, but just in case I would appreciate suggestions on the appropriate technology to identify this without too much cost involved. In photo forums they mention that the reflectivity is different on the two sides and of course this could be measured and compared for the same area of the slide, but I wonder if there is a better/best way? Any ideas welcomed.

If you wet your finger and wipe it across the slide the emulsion side feels sticky. Of course, the slide may not be much use after that!
Seriously, though, checking reflectance at an oblique angle should be possible using a focussed light source and suitable detector.

Not too familiar with optics and electronics. Would this be appropriate, one on each side of the slide? https://www.pololu.com/catalog/product/959