Experimental methods are presented for investigating molecular and microstructural aspects of the rheology of polymer melts. Measurements of stress, birefringence, and dichrcism are made simultaneously so that the connection between macroscopic viscoelasticity and the dynamics of the fluid microstructure can be established as directly and unambiguously as possible. We show how quantitative characterization of departure from the stress-optic rule can expose distinct molecular and microstructural dynamics in complex polymer melts. This general approach is applied (1) to resolve the contributions of distinct species to the macroscopic stress, illustrated using a miscible polymer blend, (2) to discriminate between dynamical regimes that are controlled by conformational or microstructural relaxation, illustrated using an ordered block copolymer, and (3) to distinguish the dynamics of topologically distinct parts of a given molecule, illustrated using a side-group liquid-crystalline polymer.