The invention concerns a method for determining the particle size distribution (PSD) of bridging agents in fluids, particularly aqueous fluids used in hydrocarbon recovery such as drilling fluids, drill-in fluids, completion fluids, and the like. It was discovered that the PSD could be selectively determined for calcium carbonate as a bridging agent because it auto-fluoresces. The method is reproducible and is not bothered by the presence of other particles such as drill solids which interferes with conventional methods, for instance, light-scattering techniques. The light used to fluoresce the bridging agent may be filtered to a particular frequency if there are other components present that fluoresce. Flow cytometry is another technique that could be used to implement the invention.