How can I take the placement test again?
I skipped the placement test, but how could I take it after I started the course?
Like Lenkvist said, if you reset your progress in your settings then you should be able to do a placement test again. If you want to know your progress, but you don't want to risk losing your levels etc - then you can take the Dutch progress quiz located in the store: https://www.duolingo.com/show_store - this won't act as a placement test but you can see how you're doing on a rough estimate :)
You only can take this progress quiz when you at a certain percent, cuz I had the opportunity but my progress drop.
Maybe you will get another chance if you reset your course? Otherwise you could try to test out of the lessons by using the locks in your tree.