Polymer coating layers are essential for the handling and proper function of optical fibers in their service environment. The analysis and monitoring of the elastic characteristics of coating layers are important for materials research and development, quality assurance, and maintenance. Most measurement protocols are destructive, require specialty samples, and may only be carried out offline. In this work, we monitor the velocities of dilatational acoustic waves in several coating layers of standard fibers, using forward Brillouin scattering processes. The measurements are non-destructive and performed over working fiber. Velocities are measured in three polyimide-based coating layers as functions of temperature up to 220 °C. Thermal changes in velocity are identified with 1% precision. The results suggest that the incorporation of nanoparticles within the polymer coating matrix improves its thermal stability.