One way of assessing student understanding is through a pen-and-paper test. It is important to include questions that assess both understanding and skills.
You can download a congruence and similarity test. This test has a variety of styles of questions. Some require written explanations and others require students to use the symbols and language of mathematics.
The task also addresses all four of the proficiency strands: Understanding, Fluency, Problem solving and Reasoning.
Student responses will provide a clear indication of the direction for the feedback to them and will also give information for planning future learning experiences.