Over time bed bugs will stain a mattress. You will start to notice little circular spots, which are slightly bigger than the full stop at the end of this sentence. Usually, you will find them in little clusters. When they are fresh they will look reddish-brown. But they dry to a dark black. Often, if you rub the area with a dampened cloth you will create a reddish smear. These little spots are their feces. If there are a lot of bugs you might also notice what looks like yellowish dust. This is their eggs and empty shells.