See CMS 6.111 for general guidance, which includes references to other CMS sections on quotation mark usage.
In British English material, we use the US style of double quotation marks, as per the Economist Style Guide.
Use quotation marks only if a question or statement is, in fact, a real quote or a hypothetical quote:
Example: Kennedy said, “Ask not . . .”
Example: The portfolio manager might ask, “How far do you want to go with this approach?”
Do not use quotation marks for rhetorical questions:
Example: If someone walks away from the deal, the issue is why. What happened?
Example: We see the markets moving wildly. What’s the cause?