Kiss Kiss Bang Bang

At a Los Angeles party, Harry Lockhart recounts recent events. Fleeing a botched burglary, Harry's friend is shot, forcing Harry to evade police by ducking into an audition. Harry unintentionally impresses the producers with an outburst of remorse they mistake for method acting. At a Hollywood party before a screen test, Harry meets private investigator Perry van Shrike, hired to give Harry on-the-job experience for his role, and party host Harlan Dexter, a retired actor who recently resolved a ten year feud over his wife's inheritance with his daughter, Veronica. He also encounters his childhood crush Harmony Lane, but wakes up in bed with her hostile friend. He runs over to Harmony's place at 6am to apologize.
Perry and Harry witness a vehicle being dumped in a lake and are spotted by the apparent killers. Perry shoots the trunk lock in a rescue attempt, but accidentally hits the female corpse inside as well. They cannot report the body because it appears Perry killed her.
Harmony contacts Harry, explaining that her sister Jenna came to L.A., stole Harmony's credit cards, and later killed herself. Believing Harry is a detective, Harmony asks him to investigate Jenna's death as a murder. After she leaves, Harry discovers the lake corpse in his bathroom and a planted pistol. Harry and Perry dump the corpse, later identified as Veronica Dexter by police. Harry discovers it was Harmony's credit card that was used to hire Perry to come to the lake, tying Jenna to their case. He goes to see Harmony, who (accidentally) slams the door on his finger, cutting it off, and she takes him to the hospital.
They go to a party but Harry is abducted, beaten and threatened to cease the investigation by the killers from the lake, and then let go. While taking Harry back to the hospital, Harmony sees the killers heading to Perry's stakeout. Realizing that Perry is heading into a trap, she leaves Harry in her car and rushes off to the stakeout, where she saves Perry and one of the killers gets shot to death by a food-cart operator. A pink-haired girl, affiliated with the killers, steals Harmony's car and unwittingly drives an unconscious Harry to her safe house. The remaining lake killer arrives and shoots her; Harry recovers the pistol and shoots the thug. His finger has fallen off again so he puts it on ice, but a dog jumps up and eats it. Harmony meets Harry at his hotel where she reveals she had told Jenna that Harlan Dexter was her real father. They quarrel in bed, when Harmony reveals she had previously slept with Harry's best friend, and he throws her out, being careful not to slam her fingers in the door.
After Harmony disappears following a lead, Harry and Perry investigate a private health clinic owned by Harlan Dexter. Perry realizes Veronica Dexter was incarcerated there by Harlan so an impostor could drop her court case. The pair capture a guard to question him but accidentally kill him, and then are captured by Dexter. He reveals he now plans to cremate his daughter's corpse to remove any remaining evidence. Harry calls Harmony, who had not actually disappeared, but had simply gone to work. Harmony steals the van containing the coffin. Harry and Perry escape, but Harmony crashes the van. Perry is incapacitated in the ensuing shootout; Harry manages to kill Dexter and his thugs, but is also shot.
Waking in a hospital, Harry finds that Perry and Harmony are fine. Perry reveals Harmony's sister was not murdered but committed suicide. Jenna had escaped incest in Indiana and located Dexter, believing him to be her real father. She accidentally witnessed Dexter having sex with Veronica's impostor, the pink-haired girl. Believing this father was also incestuous, Jenna commissioned Perry and committed suicide.
Perry travels back to Harmony's home town and confronts Jenna's father who is now bed-ridden, slapping him multiple times and calling him an animal. When the old man berates Perry for attacking a helpless old man, Perry says simply, "Yeah. Big tough guy," and leaves without further explanation.
Having lost the movie role to Colin Farrell, Harry gets a job working for Perry.

Question: Harry discovers it was what that was used to hire Perry to come to the lake?
Answer: Harmony's credit card.