We can shine electromagnetic waves through a sample and if the size of the waves is comparable to the size of what interests us, something can happen that might help us get to an answer.
Spectroscopy can be used to measure concentrations of something in a dissolved solution.
Spectroscopy can answer questions that help lead us to the identity of a molecule.
We hesitate to make it sound as if one Spectroscope is going to give you enough information to identify an unknown. However, if you know what was used to make something and you have reasonable judgement as to what the products could be, Spectroscopy might be able to confirm that you got what you wanted.
Rob Sterling 