The segregation and agglomeration of nanoparticles dispersed in polymer matrices play important roles in nanocomposite performance. A method of rapid and simultaneous visualization of macroscopic and submicron particle dispersion properties is presented, based on nanoparticle luminescence induced by europium doping. The luminescence intensity of polymer composites containing Eu-doped TiO₂ nanoparticle catalysts varied with the nanoparticle agglomerate size between 90 nm and 10 μm, and with concentration variations from segregation. These variations were detected by photoluminescence spectroscopy and visible-light photography, making this a facile characterization method for bulk composites without affecting the nanoparticle performance.