Peeper (1975)

“Back in '47, a gun was a roscoe, a private-eye was a Peeper, and murder was okay as long as nobody got hurt. In fact, anything was okay with this Peeper on the case because he wouldn't know who-done-it even if he done it himself.”

A detective is hired to locate a girl adopted 30 years earlier whose birth father wants to bequeath her his fortune.