Just set up a template - a list of rectangular regions where you will look. Each region will have a top row, bottom row, left column and right column. Then get the mean of the image there:
subImage = grayImage(topRow:bottomRow, leftCol:rightCol);
meanValue = mean(subImage(:));
If the mean is lower than some value, it's dark, if not, it's bright. That should be plenty of information to complete the project.
