On the other hand, one of the most influential essays I've read on Hacker News: https://sivers.org/below-average raises the question of why would anyone not want to be a beginner.
Norvig puts it perhaps more palatably: http://norvig.com/21-days.html
Good luck.
For your own purposes, apply for jobs (or try out projects) that seem out of your depth. Break them up into chunks and Google your way through them.
One thing you could is step through the more advanced O'Reilly books for Python, or whatever topic, and see how much you already know. They happen to have quite a large collection of good Python books.
I had the same questions as you, and I confirm those labels are subjective, so I stopped feeling not experienced enough.