If looking at an x-ray, what tells a dentist whether you need a root canal?

Disease: Large decay, infection around the end of the root, among other clinical input the dentist obtains.

Xray or pulp test: A root canal is indicated after making a proper diagnosis. This is sometimes possible with just an xray if it shows clear pathology or infection. It may show deep decay or infection at the root apex. Other times it is diagnosed after performing pulp tests to determine the health of the pulp. Or it may be done visually by looking in the mouth when there is clear infection into pulpal tissue.

It's complicated...: Most times we need more than an x-ray to find the rationale for root canal treatment. A dark area at the end of the tooth within the x-ray may suggest the need for a root canal treatment. A very deep dark area within the crown portion of the tooth may point towards the need for a root canal. However we use signs, symptoms, and a visual exam as well to aid us in the final rationale for care.