Paper describe researches performed to develop a method for checking the light transmission through plastic scintillating fiber. Such fibers are used in different industrial or research application but for our purpose are main components of a flexible detector. Such detector use photon scintillating signals read by higly sensitive detectors and special measures taken to get such sensitivity. We present in paper a method and algorithm developed for measurement of light transmission through plastic fiber when signals are at level of nW. We develop a stand to implement and verify this method. We use fibers with connectors at each fiber end. Fibers are previously polished at each end in order to allows to pass the signal through surface with low losses. Using the stand we perform measurements to analyse the feasibility of method. We present the results that demonstrate the feasibility of method. We succeed to measure signals at level of nW using special measures. Our results shows that method and stand can be used to measure low level signals through fibers. Using this method, it is possible to check the quality of plastic fiber polishing. A fiber with end surface well-polished allows to pass the signal at such low level (nW).