According to Wikipedia, glucose monitors have sensors under the skin. Seems a little invasive to do as a screening test.

I wonder if you could use spectroscopy to analyze glucose content in blood? For example, take an existing oxygen meter design and tune it to a different wavelength to detect glucose instead of oxygen.

The sensor has a small thread that is completely painless actually! Not invasive at all. Beside the small device stuck to the skin, you cannot tell you have the sensor on at all.