Casual:
* Private eye: This is a classic informal term for a detective, often used in film noir or crime fiction.
* Shamus: A slightly more old-fashioned term, also common in crime fiction.
* Dick: This is a slang term for a detective, and can be considered somewhat offensive.
* Snoop: This term suggests someone who is nosy or inquisitive, but can also be used to describe a detective.
* Investigator: This is a more general term, but can be used informally to refer to a detective.
Informal and Friendly:
* The sleuth: This is a fun, playful term for a detective.
* The gumshoe: This is another classic term for a detective, often used in a lighthearted way.
* The investigator: This is a more neutral term, but can be used informally.
* The caseworker: This is a more formal term, but can be used informally to refer to a detective.
Slang:
* Cop: This is a general term for a police officer, but can sometimes be used to refer to a detective.
* Flatfoot: This is a slang term for a police officer, but can sometimes be used to refer to a detective.
The best way to choose an informal term for "detective" is to consider the specific situation and the tone you want to convey.