I lead a lot lately and the one time I'm inclined to pull over and look for the group (be that 1 or several riders) is when I can usually see others behind me, but suddenly nobody is there. Unless I just passed somebody and they didn't get around yet (I usually try to wait for others behind me to get around regroup), they should be there and disappearing is bad.
If you get spread out quite a bit, and are riding in a place with other riders, its also a good idea to be sure the next rider in your group is the one you see, not just another rider possibly with similar bike/gear. As Doug said, "May be a good time to regroup." And sometimes you just need to do that, to avoid becoming more spread out.