A detection apparatus enables detection of a substance that may be present on the surface based on measurement of a stream probe signal correlating a substance at least partially obstructing the passage of fluid through a stream probe includes a distal optical gum detector transmission portion and a distal optical gum detector reception portion in a position to transmit and to receive, respectively, an optical signal controlled by a controller, enables the controller to determine if an open port of a distal tip of the stream probe is in contact with a substance at least partially obstructing the passage of fluid through the open port and not in contact with the gums of a subject or of a user of the detection apparatus to override false positive signals.