An embodiment of the present disclosure seeks to select characteristics of incoming intensity data that cause comparisons of selected characteristics to produce defined probe off space having reduced crossover with defined probe on space. Once defined, the present disclosure compares characteristics of incoming intensity data with the now defined probe off space, and in some embodiments, defined probe on space, to determine whether a probe off condition exists. When a processor determines a probe off condition exists, the processor may output or trigger an output signal that audibly and/or visually indicates to a user that the optical sensor should be adjusted for a proper application to a measurement site.