when does louis find out about mike

Louis Litt finds out about Mike Ross not attending Harvard Law School in the TV series "Suits" in Season 4, Episode 10, titled "This Is Rome." Louis figures out the secret when Mike fails to recognize a key that is given to students who graduate magna cum laude from Harvard Law School, which leads Louis to investigate Mike's background further. This discovery prompts Louis to confront Jessica Pearson and leverage the information to negotiate his position in the firm. This moment is significant because it reveals the fraudulent nature of Mike's credentials, which had been a central secret in the series until that point.