We describe a simple experimental tool that enables stability of multicomponent nanoparticle suspensions to be readily assessed by establishing a confinement-imposed chemical discontinuity at the interface between co-flowing laminar streams in a microchannel. When applied to examine Al₂O₃ nanoparticle suspensions, this method readily reveals compositions that are susceptible to aggregation even when conventional bulk measurements (zeta potential, dynamic light scattering, bulk viscosity) suggest only subtle differences between formulations. This microfluidic stability test enables simple and rapid assessment of quality and variability in complex multicomponent mixtures for which few, if any, comparable data exist. The paradoxical ease at which localized aggregation can be triggered in suspensions that would otherwise appear stable also serves as a caution to researchers undertaking tracer-based studies of nanomaterial suspensions.