Before the advent of modern spectrometry techniques (NMR and mass spectrometry), there was a compendium of tests to suss out what sort of things were hanging off a molecule. You took your stuff, added some eye of newt, and if black (but not white) soot rose up, you knew you had an arylamine (or at least had some evidence you did). You still see them occasionally in sophomore Organic chemistry labs, but they’re going by the wayside, too.
One such reagent was naphthoresorcinol:
This one will give you a red adduct with an aldehyde in acidic solution. Anyone know the pathway or care to guess??