Do a histogram or intensity threshold on the entire image. With all telltales off the maximum pixel intensity should be significantly lower than with any or all telltales on.
I know your method,but I want to get the coordinary of the telltale which has be turned off,and then to test the intensity.
I have ever used other Vision tools named "Gips Vision",its method is to learn the test files when all telltale on ,and auto made one new test file to test the condition when all telltales off,and the new test file contains all the telltales coordinary information.
I am guessing that you meant coordinates. Hope the examples below help!