A flow sensor for detecting liquid flow generated by a spray device is non-invasive and capable of responding to rapid changes in the flow. The flow sensor includes a light emitter and a light detector, which may be disposed adjacent the liquid outlet of the spray device and oriented such that a light beam projected by the light emitter is aimed at the spray generated by the spray device and reflected by the spray into the light detector. The light detector senses the reflected light and generates an output indicative of the presence of the flow from the spray device. A deviation of the spray from the intended spray axis may also be detected.