I like the idea of exit slips, those short assessments at the end of the lesson to make sure the students understand the point of of the lesson. Because my students are with me all day, instead of waiting until the end of a work session, I had students answer one question at the beginning. As they worked independently or at stations, I could easily see who answered correctly and who needed assistance.
FYI: I'm not a fan of permanent ability groups. I like giving short assessments like these and addressing skills as they come up. Different students have different needs on different days.
To download my template for Ticket out the Door, click here.