I would first point out what I saw students learning, then provide suggestions phrased as either consider to... or continue to... and finally I would ask them open-ended questions that would encourage them to think about their learning.
I might ask them "if you could add a character to your story..." or "if you didn't have any steel wool for that experiment..." or "if multiplying wasn't an option..."
This kind of formative assessment has served me well, but when I first started out with all this, I was having my students do all this as peer and self-assessments on every little assignment. They, and I, quickly grew tired of these assessments - not because they were faulty, but because we were simply doing it too often.
That's when I learned a valuable lesson - even the best assessments can be over done.
That's also when I learned that it makes most sense to spend the majority of our time learning - actually focusing on what we are learning, and only on rare occasions would we stop and reflect upon how well we are learning.
For more about the formative assessment methods I described above, see this post on Replacing Grading.