Discovery of optical activity of enantiomers - where did the idea of using polarized light came from?
It wasn’t a standard lab procedure to test the effects of polarised light on a certain compound/solution/whatnot. However, back in the day it was standard lab procedure to re-run a certain experiment with practically everything your lab had: water, acids, solvents, table salt, cat hair, human hair, dust, fabric, … You would note if you observed an interesting effect and, where applicable, if that interesting effect would only occur in some cases.

So once you are aware that something like Iceland spar produced interesting phenomena (even though you might not be able to explain these phenomena just yet) you would start trying out all sorts of things to test what this interesting optical phenomenon is capable of doing. By chance, you would run across something that you would be able to observe as cancelling of light, even though all components of the system would let light through — and then you would attempt at understanding what you just observed.

They say science is merely grown men playing around for a reason.