We used to be big fans of using clickers with our smartboard to quiz students during class. For example, after a lesson about diabetes, it would make sense to ask the class after a diabetic patient scenario: "Would his insulin levels be going up or down?". Using the clickers, the students can answer. If one student sees that only two people answered the wrong answer and he/she was one of those people, it gives them a push to study a little harder without having to publicly admit to the wrong answer. It's also a great way to get students participating after a long lecture.
But not everyone has a smartboard and not everyone has clickers. So we found a great alternative! Poll Everywhere allows up to 40 people respond to any given question via text message for free. Instead of trying to get students to put away their phones, encourage them to take them out and text the answer to a question during class.
That way you can see, in real time, if your students understand a concept or not.
There are also payment plans for schools but I'd imagine you could work around that with the free plan which allows 40 responses per poll.