What does one say or do if they think that a person's idea, behaviour, etc. is crazy to their mind?

For instance:
In Germany, when someone dangerously passes you, you will show them by tipping your finger on your forehead(or temple) that they "has a bird" _{(literal translation of "einen Vogel haben")}. This action is called "jemandem den Vogel zeigen", literally translated: "to show someone the bird".

I found the phrase "bats in the belfry" which has a similar meaning to the German phrase. I haven't heard it before, and I'm not sure if this phrase is in common use.

What do you usually say? Do you also have a gesture like tipping your finger on your forehead/temple? And is it insulting by law?

I'm interested in both American and British phrases and gesture.

Just as side note: In Germany it is highly controversial if it is an insult by law or not. There were several court decisions pro and contra.