The notion of “reasonable indicium” plays a pivotal role in three core domains of criminal adjudication: establishing the truth in modern offences such as cybercrime and complex financial crime; strengthening the judge’s evidentiary assessment in traditional offences, including those subject to fixed statutory punishments; and scrutinizing the veracity of claims throughout the trial process. For its evaluation, three categories of criteria are proposed: formal criteria (legality, relevance, and admissibility), substantive criteria (inferential strength, sufficiency, and coherence), and procedural criteria (possibility of expert assessment and testability). Using a descriptive analytical method and a comparative critical approach, this study examines the conceptual gap and the absence of clear standards for “reasonable indicium” in the criminal justice system of Iran. It shows that although the term is envisaged in provisions such as Article 114 of the Code of Criminal Procedure and Article 156 of the Islamic Penal Code, its inherent vagueness has led to divergent interpretations, discretionary decision making, and ultimately the weakening of the fundamental principle of the presumption of innocence. By contrast, other legal systems have developed criteria such as “observable and articulable conduct,” “probable cause,” and “serious or corroborated indicators.” The study concludes that, in the absence of transparent and binding criteria, “reasonable indicium” will remain an ambiguous and contested construct. To address this problem, it recommends the adoption of a unified judicial guideline that precisely defines formal and substantive standards, and the establishment of binding benchmarks such as “a minimum evidentiary sufficiency threshold” and a “mandatory independent expert evaluation” as preconditions for the acceptance of any reasonable indicium.