I remember William Herschel was the first to discover infrared radiation, by observing that, when splitting sunlight using a prism, the dark area next to the red light would warm a thermometer. Wikipedia suggests that we knew about the photoelectric effect and that methods for turning infrared photons into electricity existed.

You had Maxwell's equations in the 1860s, so you'd have the mathematical abstractions of electric and magnetic fields, and that they work together to make light waves. Before that, scientists knew that an electric current would turn a compass, so you could use a magnetized material as a current detector (and you start to get into the basics of measuring the electronic and magnetic properties of something).