Okay, so it's a common joke that petrologists play on their students -- there among all the rock seriousities that are given to students to identify, there is always the stray bit of asphault. But then, you really should be able to tell asphault from real rocks, so here it is.
Actually, it's not always an easy call. If fresh, the tar might still be gooey. Try putting a piece of the stuff in water then sniffing it to see if it smells like asphault. It this doesn't work, apply a match and see if it melts or burns. If not, your guess is as good as mine.