A method comprising: receiving, as input, a video sequence comprising a plurality of frames and depicting a scene; detecting, in each of said frames, one or more regions of interest (ROI) associated with an object in said scene; extracting, from each of said ROIs with respect to all of said frames, a feature set representing spectral reflectance; at a training stage, train a machine learning classifier on a training set comprising: (i) all of said feature sets, and (ii) labels indicating whether each of said ROIs depicts living skin tissue; and at an inference stage, applying said trained machine learning classifier to a target feature set extracted from a target ROI detected in a target video sequence, to determine whether said ROI depicts living skin tissue.