Abstract: Apparatus and method of determining presence or absence of light activity in an optical fiber without separation or disconnection thereof from other fibers or from apparatus to which the fiber is connected, and without interruption of transmission. The method involves contacting the fiber with a probe and moving it laterally to produce a macro-bend at which a portion of light traveling through the fiber, if any, escapes from the fiber. The escaping light is reflected from a concave surface in a fixture through which the fiber passes and is detected by a photocell in the probe which contacts and moves the fiber. The probe is engageable with the fixture in either of two rotational orientations to receive light reflected from two different areas of the reflecting surface, thereby indicating the direction of light travel. Initial calibration and positional adjustment of the probe relative to the fixture also permits determination of light intensity.