The idiom 'a smoking gun' is used to talk about something which is considered strong circumstantial evidence or good proof of something. 'A smoking gun' refers to circumstantial evidence which seems very convincing.

1. The police investigation has not yet produced a smoking gun to help them secure a conviction in the case.
2. The CCTV footage was a smoking gun proving that he had been at the scene of the crime.

1. The missing document I tracked down is the smoking gun that proves Anna has been embezzling funds from the business.
2. The accounts were the smoking gun that led the company to be accused of tax evasion.