Ask HN: Tools for checking my knowledge?

Do you know what tools or services exist for me to test my knowledge of certain programming languages? I am confident in my C++ and Python, but am always looking for holes in my understanding, and also for staying sharp.

I regularly do interview problems + side projects but this focuses on more on algorithmic and problem solving skills, and not necessary language features.

For example, the Triplebyte quizzes for specific languages, but I'm unsure if I can take that without being contacted by recruiters.

Thanks!