Write About It! One very quick way to see if students understand a new concept is to have them solve a problem and then write a short paragraph describing how they solved it before they leave class that day.  Look over the answers after class and then divide the students into two groups:  those who got it and those who didn’t.
For the first ten minutes of class the next morning, you can pair the kids who got it with the ones who didn’t and ask the first group to explain the concept to the second group.
Then, either right away or at the end of the period, give them another problem covering the same concept and have them solve it and include a paragraph on how they solved it this time.  Hopefully, most, if not all, will understand the concept now.