NMR spectroscopy
An NMR diagram displays an x-axis with units of parts per million, or "shift." Peaks that occur farther to the left in the diagram (these peaks are said to be shifted downfield) represent hydrogen atoms that are closer to electronegative atoms like oxygen or nitrogen, or connected to an aromatic (benzene) ring. Hydrogen atoms that are adjacent to other hydrogen atoms can split peaks as well. If there are n hydrogen atoms "next door" to a single one, that single one's peak will be split into n+1 peaks. For example, if a hydrogen atom is adjacent to three others, that hydrogen atom will be split into 4 smaller peaks, called a quartet. Hydrogen atoms that are part of O-H or N-H bonds won't be split, but they will appear on the diagram as a rounded lump, called a broad peak. By looking at all these factors in a diagram, a chemist can figure out what compounds are in their sample, and they can determine if the compound they have made is pure.