How can you tell if you're idealizing a person? What if things are just good naturally?

Today, L told me that she wants me to trust that it's okay to be angry at her. I have had no major issues with L. No fights, ruptures, and only one misunderstanding. And I got frustrated only once because she didn't answer a question. I accept her and her boundaries. She hasn't done anything that I'd get mad about.

How do I know if I'm idealizing her or is it just reality?