If you click on the courses part at the top left, there will be a percentage of mastery (if applicable). Otherwise just do 100% on all the small exercises and watch all the videos to have that particular mastery. 

Hi, Art3mis!

We don't have a feature to check all of the skills mastered in your account at the moment.

But we are always working to add features that improve our site, and we love getting input from our community.

I've passed your feedback along to the team and it will be part of our conversation as we plan the future of Khan Academy.

Guilherme
Community Support

Hi Art3mis,

I have some great news to share with you!

Based on feedback that you and other learners shared with our team, Khan Academy has recently redesigned the Course and Unit Mastery pages and introduced an overview of the skills in a single course so you can tell in a glimpse what skills you’ve mastered and those that still need to be completed!

To know more about this change, please take a look at the article Update: New mastery progress visualization on Course and Unit pages.

Most importantly, we want to thank you for taking the time to share your feedback to help make Khan Academy a better learning resource for everyone. We take all feedback to heart and appreciate you sharing your ideas with us.

Thank you so much for helping us improve Khan Academy!

Kind regards,