Peaks of each chemical is unique so you can scan database of known chemicals and compare.

You can even measure the amounts of various chemicals in solution, but that would require complex calibration. In this case you are basically writing ML system that is trying to fit various peak heights/areas to amount of compound.

This doesn't really explain how but it does talk about the uses of an FTIR in a drug checking, harm reduction situation.

https://globalnews.ca/news/4540315/new-drug-checking-machine...

Not an expert, but if you can test for contamination why wouldn't you be able to test for the drugs themselves?

Raman spectroscopy[1] is one that I know of that is frequently used to determine chemical composition. Spectroscopy in general still amazes me.

[1] https://en.wikipedia.org/wiki/Raman_spectroscopy#Application...