It's important to periodically check for understanding during a lesson, but often a teacher gets blank stares instead of helpful feedback. I heard of making these student "whiteboards" from a colleague, and it's made all the difference. Simple insert a white piece of paper or card stock in a page protector. Then supply dry erase markers (or have students provide their own) and a paper towel, and you have portable, erasable boards. When I'm explaining a concept, I'll often stop and ask an on-the-spot quiz question and have students write their answers on their boards and hold them up. This way, I can see at a glance how many are getting it. Better yet, it engages the students and most happily participate.
