Two reasons. First off, Mimi Wynant came across with the watch chain she plucked off Julia's body. It belonged to Clyde Wynant.
10 seconds sound clip from the The Thin Man (1936) classic radio play.
You can hear this line at 00:33:14 in the radio play.
Quote context
[...]
- This is Al Nunheim again. You know, I called you last night.
- Hey listen, are you still interested in that proposition? You are, huh? All right then.
- Now, here's the dope and get this straight, the man who killed Julia Wolf is... (Gunshot)
- What's this Guild? You think Wynant killed Julia Wolf and Nunheim?
- Right.
- Why?
- A pip!
- The bullet that killed Nunheim came from the same gun!
- That's all right, Inspector.
- Clyde Wynant is guilty of both of those murders.
- Maybe.
- What?
- Fifty'll still get you a hundred.
[...]