Substances on or adjacent a tubular disposed in a subterranean wellbore can be detected and/or their presence identified by monitoring gamma rays scattered from the substances. The substances include deposits on a sidewall of the tubular and liquid contained in the tubular. Examples of the deposits include asphaltenes, paraffins, scale, sand, and the like. Gamma rays are strategically directed from a tool disposed within the tubular and to adjacent a sidewall of the tubular. Some of the gamma rays scatter from the substances adjacent the tubular and are detected with detectors set a designated axial distance from the gamma ray source.