Short answer: In Smallville, Lex learns Clark’s secret in Season 3, during the events surrounding the episodes Shattered and Asylum. Details and context:
- Lex discovers clues about Clark’s abilities and extraterrestrial origins starting in Season 2 with the symbol-driven investigations of the Kawatche Caves, but his explicit realization comes in Season 3 as the tension between them escalates and Clark’s powers and Kryptonian connection become harder to ignore. This leads to Lex confronting Clark and, in the narrative arc of Shattered and Asylum, Lex’s understanding of Clark’s secret is effectively confirmed, altering their friendship and setting the stage for the later breakdown of trust.
- The broader arc shows Lex gradually piecing together Clark’s truth: the cave inscriptions, Clark’s meteor-powered abilities, and the influence of the Kryptonian lore around Smallville all contribute to Lex’s eventual realization, culminating in the Season 7 payoff where the character explicitly references Kal-El in moments of confrontation.
If you want, I can pull exact episode-by-episode moments or quotes that mark the key turning points where Lex deduces Clark’s secret.
