Question

What is the punctuation mark that shows you forgot to indent a paragraph?

Answer 1

The mark is in the graphic below.

Explanation:

I'm not sure if you are looking for the "new paragraph" mark or the "insert space" mark, but both are here.

If these aren't what you are looking for, try googling "images editing marks". I chose a relatively simple graphic - there are more complete ones in the Google search.