Fun way to test your skills after finishing a course.
I had the best idea on how I could test myself after finishing the courses I'm currently working on! I could find the flipped version (if available) Ex: Since I'm taking Norwegian, I could see if there's an "English for Norwegian speakers" course, and take it. I think doing this would strengthen my skills and test how much I really know.
The Duolingo term for that is reverse tree. Learning a 3rd Language from your 2nd Language is called laddering
Many people have reported success with that very method. I haven't gotten that far yet myself, but might do it when I'm finished with my tree