How to Know the Knowing of Anicca, Dukkha, Anatta
There is knowing of Anicca, knowing of Dukkha, and knowing of Anatta, separate from each other. And then there is knowing of all three together, or rather the knowing of vision behind these three designations, in all its implications.

In my experience, it is seeing all implications, top to bottom, is what makes all the difference in the world. I had intellectual understanding of the three marks for years. I even understood shunyata, or thought I did. I even knew that Enlightenment can't be a state. And I still missed the point.

It's all about implications, of all three, together. When you know reality behind the three, and all implications of this reality, then you can be sure you know what is there to be known. Until then, to think "anicca (etc.) is yet to be grasped" would be a valid thought. Why? Because if you don't see how it fits together, or do see how it all fits together but not its implications -- then your knowledge of individual components is obviously not 100% complete yet.