Try Kahoot! to assess understanding in your lesson.
Go to http://www.getKahoot.com and sign up for a free account.
You can create a game based classroom response activity, where learners are encouraged to use their mobile devices. You can post up a series of questions on the board which learners interact with and provide responses, via their mobile devices. Try putting the learners in charge and have them ask the questions of their peers.
(Image from www.getkahoot.com)