The Untouchables

During Prohibition in 1930, Al Capone has nearly the whole city of Chicago under his control and supplies illegal liquor. Bureau of Prohibition agent Eliot Ness is assigned to stop Capone, but his first attempt at a liquor raid fails due to corrupt policemen tipping Capone off. He has a chance meeting with Irish-American veteran officer Jimmy Malone, who is fed up with the rampant corruption and offers to help Ness, suggesting that they find a man from the police academy who has not come under Capone's influence. They recruit Italian-American trainee George Stone (aka-Giuseppe Petri) for his superior marksmanship and intelligence. Joined by accountant Oscar Wallace, assigned to Ness from Washington, D.C., they conduct a successful raid on a Capone liquor cache and start to gain positive publicity, with the press dubbing them "The Untouchables." Capone later kills the henchman in charge of the cache as a warning to his other men.
Wallace discovers that Capone has not filed an income tax return for some years and suggests that the team try to build a tax evasion case against him, since he is well-insulated from his other crimes. An alderman offers Ness a bribe to drop his investigation, but Ness angrily refuses it and throws him out of the office. When Capone gunman Frank Nitti threatens Ness' family, Ness has his wife and daughter moved to a safe house. His team flies to the CanadaâUnited States border to intercept an incoming liquor shipment, aided by the Royal Canadian Mounted Police, killing several gangsters and capturing George, a Capone bookkeeper. Malone shoots a gangster through the mouth (not revealing the man is already dead) to scare George into agreeing to testify against Capone.
Wallace prepares to escort George from the Chicago police station to a safe house, but they are shot and killed by Nitti, who has infiltrated the station. Ness confronts Capone and his men over the deaths, but Malone intervenes to save him from being killed and urges him to persuade the district attorney not to dismiss the charges against Capone. Realizing that police chief Mike Dorsett sold out Wallace and George, Malone forces him to reveal the whereabouts of Walter Payne, Capone's chief bookkeeper. That night, a knife-wielding thug sneaks into Malone's apartment; Malone chases him out with a shotgun, but falls victim to Nitti's Tommy gun ambush. Ness and Stone arrive at the apartment; before dying, Malone tells them which train Payne will take out of town.
At Union Station, Ness and Stone find Payne guarded by several gangsters. A gunfight breaks out on the lobby steps, resulting in all the gangsters being killed and Payne being taken alive. As Payne testifies at Capone's trial, explaining the untaxed cash flows throughout the syndicate, Ness notices that Capone seems unusually relaxed and also spots Nitti carrying a gun under his jacket. Ness has the bailiff remove Nitti and searches him outside the courtroom; a matchbook in Nitti's pocket contains Malone's address, leading Ness to realize that he killed Malone. Nitti shoots the bailiff and flees to the courthouse roof. Ness has the opportunity to kill Nitti at one point but chooses not to, but when Nitti mocks the way Malone died, Ness pushes him off the roof to his death.
Stone gives Ness a list, taken from Nitti's jacket, that shows bribes paid to the jurors. When the judge refuses to consider it as evidence of jury tampering, Ness bluffs him into thinking that his name is in Payne's ledger of payoffs. The judge subsequently orders that the jury be switched with one in another courtroom, prompting Capone's lawyer to enter a guilty plea on his behalf. Capone is later sentenced to 11 years in prison. Ness closes up his office and gives Malone's St. Jude medallion to Stone as a farewell gift. As Ness leaves the police station, a reporter mentions a rumor that Prohibition may soon be repealed and asks what Ness will do it if that happens. Ness replies, "I think I'll have a drink."

Question: Who discovers that Capone has not filed an income tax return for some years?
Answer: Wallace