A monitoring system and a method for monitoring a reverse osmosis (RO) membrane in an RO unit is capable of detecting the formation of mineral salt crystals on the surface of the RO membrane. The monitoring system includes a reverse osmosis monitoring cell coupled to the RO unit so as to receive a sample stream taken from either the feed stream to, or the concentrate stream from, the RO unit. The cell has a visually observable RO membrane that is visible to an imaging system that creates and collects images of the visually-observable RO membrane, and that conveys an image data signal to a data processing system that is operable to translate the image data signal into visual images for display, and to correlate the data in the image data signal with a scaling condition on the RO membrane in the RO unit.