Taking an extra radiograph with altered mesial or distal horizontal angulation is the most reliable method to identify an additional root canal because it provides a different perspective of the root canal system, revealing canals that may be superimposed or hidden in standard periapical views. This technique improves visualization of complex root anatomy by minimizing overlapping structures and enables accurate detection of accessory canals. Other methods, such as thermal tests or magnification alone, do not provide clear anatomical confirmation of an extra canal’s presence. Therefore, multiple angled radiographs are essential for precise canal identification during endodontic treatment.

Reference: Cohen’s Pathways of the Pulp, 12th Edition.