Scaling remains a serious barrier to membrane-based desalination. While some real-time techniques have proven effective for scaling detection, none provides the crucial capability of chemical identification with high temporal and spatial resolution under realistic operating conditions. In this work, we describe a real-time technique for detection of early stage scaling via chemical quantification using Raman spectroscopy. Experiments utilized a custom bench-scale flow cell integrated with a commercial Raman microscope that accesses the membrane through a 10 mm optical window. Calcium sulfate was used as a model foulant at high concentration (1.8 g/L) to minimize the time required for membrane scaling. The experiments were conducted with a commercial brackish water reverse-osmosis thin film composite membrane operating at a feed pressure of 1.17 MPa (170 psi) and a feed flow velocity of 4.7-5.6 cm/s. Raman measurements were made in real time at a laser excitation wavelength of 785 nm. Real-time results were validated with post-mortem SEM and energy-dispersive X-ray analysis and indicated the capability to detect early-stage scaling characterized by a relatively modest reduction in the permeate flow rate.