Solar cell modules are connected in serial and parallel in a photovoltaic system. All the solar cell modules degrade over time and the most degraded module in a string decides the output level of the string. In this study, we suggest a heath monitoring method which enable to detect the most degraded module in a string without separating the module from the string. We suggest two method; first placing a non-transparent film and second illuminating an extra light on a module in a string to make an artificial shading or enlightening effect, respectively. It needs to monitor the current-voltage curve of the string while placing the film or lighting to the next one. We show analytically that the most degraded module can be detected by comparing all the string current-voltage curves. We demonstrate the method for an outdoor photovoltaic string.