If you go to any Ed Tech conference, the clicker vendors are everywhere. They're a good way to check student comprehension, if the content supports multiple choice type questions.
They are, of course, expensive gadgets.
It occurred to me the other day (while brainstorming at a tech conference) that you could use Google Forms to create a quiz students would take as the class works through a review lesson (for example). When they respond to the questions, Google tabulates the results in a spreadsheet that you would have open on your computer, where you could see the percentage of right/wrong answers, and adjust the review accordingly. A bit primitive, as you have no idea who answered correctly, but you could gauge the class's progress as a whole.