Students are assigned certain topics to teach or present to the class, just as a teacher might.
They might have to complete a maths question on the board and explain their process, or teach the rules for the verb agreements for have and has, and give examples. The teacher would observe students' explanations (and interject as needed) and could see the teaching students' understanding of the material.
This also evaluates the skills of planning, speaking and presenting.
Teachers could have the students evaluate their writing mid-unit, based on a checklist and a mini-progression, in order for them to see where they are and what they should focus on next. This can be done independently, after the students understand the check-list, and afterwards the student has a guide as to what they need to improve. Teachers can also have one-on-one conferring with the students to guide them in this process.
Other examples that could be used as formative assessments can be mid-unit reflections, exit tickets, quizzes on Kahoot, mid-uni assessment or mind maps that show their understanding of the unit at that point.