It is easy to see on most homes while the growth is in process; there are dark black streaks, particularly on the north facing roof, though once the algae has fully taken over it is sometimes hard to tell. (It's never too late to remove.) The best place to look to find out if your roof is dirty is below metal roof jacks flashing and sky lights. There you should find lighter colored streaks or patches...that is the original color and that is what your roof will look like after we have completed our service!