I don't know where I saw this, but it was recent and the discovery was useful.
There's a quick test to see if someone is overconfident. Asking someone to give you a range for something that they don't know. For example:
What year was George Washington born? If you don't know, then please provide a range in which you are 90% confident his birth year would be included.
If the respondent provides a small range that does not include the correct year, this is a sign of over confidence.
This test, at least for me, is a good example of the Testing Effect. I got my first answer wrong, but since then, have taken to widening the range for unknown answers. Or quickly admitting when I don't know.