Biocompatible hydroxyapatite dielectric is known to exhibit enhanced bioactivity when irradiated by an electron beam. The value of the trapped charge induced by the beam usually strongly differs from the total irradiated charge. Therefore, a method for measuring the real value of the charge trapped in the dielectric is developed. The value of the trapped charge is determined from the shape change of the pendant drop hanging in the proximity of the charged sample. The method is simple, non-invasive, and provides reproducible results for charges of the order of nanocoulomb. Using the method, the long-term charge stability in the biocompatible hydroxyapatite is demonstrated.