When should you be using unit tests [closed]

The internet is full of websites explaining:

• what a unit test is
• why you should unit test and
• how to unit test.

But it doesn't state anywhere when you should unit test.

I currently have a plethora of methods, some of which need unit tests and some of which do not. However, I struggle identifying which do and which don't.

• How do you determine which methods should be unit tested and which shouldn't?
• How do you differentiate between what is unit testable and what isn't?
• Is there a general guideline as to what you should be unit testing?
• Is there such a thing as overdoing it when it comes to unit testing?