How can you tell the difference between a cavity and root canal on an x-ray?

Color: A cavity appears as a dark area on the crown of the tooth, whereas a root canal appears as a white line in the root of the tooth.

Yes: Cavity is dark spot on the tooth. A need for root canal is also a dark area that is much larger , it is so large that it enters or touches the center dark line ( which is your nerve). Once the cavity has grown large enough to touch your nerve a root canal is needed to save the tooth.