Establishing a supervising engineer’s wrongful conduct in construction disputes requires distinguishing a breach of supervisory duty from defects arising during construction, although regulations do not specify every aspect of professional care in varying circumstances. This study examines whether valid engineering custom can independently guide the identification of such conduct. Using a descriptive–analytical method, it analyzes Imami jurisprudence, Iranian law, three judicial decisions, and legal approaches in England, the United States, Canada, China, and France. The findings indicate that engineering custom provides an independent standard for reasonable supervisory conduct but cannot impose duties beyond the engineer’s lawful or contractual role. Its principal functions are interpretive and supplementary; recognizing an unstated professional obligation requires evidence of a valid professional norm compatible with the engineer’s authority. Analysis of the cases shows that issuing certificates, reporting hazards, or assigning percentages of fault does not necessarily establish compliance with, or breach of, supervisory duty. The engineer’s actual acts or omissions must instead be evaluated against the care reasonably required under the circumstances at the relevant time. The study therefore proposes a four-stage framework: identifying the source and scope of duty, determining expected professional conduct, establishing actual conduct, and assessing unjustified departure from the applicable standard. This framework supports reasoned findings of wrongful conduct while revealing evidentiary shortcomings. Ultimately, establishing such conduct requires proof of a specific duty and a demonstrable departure from the care required by valid engineering custom in comparable circumstances.