A system is described for evaluating coagulation of particles in a downhole fluid-particle mixture based on dielectric measurements. An example downhole treatment is one in which flocs are used to plug a high-permeability subterranean formation zone as part of a stimulation procedure. An injection tube is positioned within the wellbore to the high-permeability zone. An instrumented section of tubing includes one or more dielectric probes that are positioned and configured to make dielectric measurements of the particle-fluid mixture flowing in the tubing or in the annulus. The downhole dielectric measurements are used to indicate whether or not the particle-fluid mixture has the desired structural properties. An operator on the surface can make adjustments in real-time according to the received dielectric measurements.