Near the completion of his sentence in Sing Sing prison, Paul Vitti's life is threatened by assassins and corrupt guards while incarcerated. He starts singing showtunes from West Side Story to get the attention of Ben Sobel, who previously hung up on him while attending his father's funeral. The FBI calls in Ben to see if Vitti is really insane. This appears to be the case, and the FBI approves Ben taking Vitti out of prison, into his own custody, for further therapy. On their way out in Sobel's car, Vitti reveals that he faked it. Needing some therapy himself after his father's death, a grieving Sobel talks Vitti into finding a regular job (as requested by the FBI). Vitti attempts to find a legitimate job (he tries a car dealer, a restaurant, and a jewelry store), but his rude manners and paranoia only complicate things further (which end up in him getting fired each time).
At the same time, Vitti is told by de facto boss Patti LoPresti that the Rigazzi family wants him dead. He responds to this by telling the Rigazzis that he is "out" and seeking a new line of employment. He eventually finds employment working as a technical advisor on the set of a Sopranos-like mafia TV series. Meanwhile, FBI agents inform Sobel that Vitti has his former crew back together, and may be planning something major. This rouses Sobel's suspicion, and he visits Vitti. Both get caught up in a car chase with Rigazzi hitmen, which ends up with Vitti escaping. The FBI blames Sobel, and gives him 24 hours to locate Vitti.
After locating Vitti (through Sobel's son Michael, who is now working as Vitti's chauffeur), Sobel discovers Vitti is planning a big armored car heist with LoPresti as a partner. He attempts to intervene and talk Vitti out of it but Vitti proceeds and Sobel is forced to go along as well. The crew ambushes the armored car with smoke grenades, and lifts it over a fence in the midst of the confusion. They extract $20M worth of gold bullion, but LoPresti's thugs take over, revealing themselves to actually have been working for Rigazzi. Sobel, in a fit of anger, beats one of them and Vitti's men take care of the rest. They use the gold bullion to frame the Rigazzi family, leaving three Rigazzi goons locked in the armored truck suspended from the crane. This leads to the arrest of the entire Rigazzi family, and in turn, prevents a mob war.
Sobel meets with Vitti and Jelly near bridges on the New York waterfront, and they part ways again as friends, singing another West Side Story showtune together.

Question: How did the FBI determined the truth if Paul was really insane?
Answer: FBI called Ben.
Alternative answer: Used a therapist.