Different strains have different leave colors. so just looking at the visible spectrum bouncing off leaves would probably not tell you much you can’t tell with the naked eye. Leaves DO turn yellow near the end, but that is very easy to see. Most of the disease and deficiencies have naked eye effects on the leaves. I would recommend a handheld 60-100 x magnifier or microscope so you can look at the backs of leaves for spider mites, aphids, or thrips.
You can also use the magnifier to look at the trichomes on the buds to tell when they are ready to harvest by color. That tells you about the THC content, but I suppose you could extract a sugar leaf into isopropanol and look for the absorption peak of THC. It will mostly be THC-A until it is baked or smoked. The literature will tell you what kind of spectroscopy to use.