Here are some posters that you could use in your classroom to check kids understanding. They could show you have they are feeling after a certain activity or part of day. It is good feedback and a great practice to have kids self-evaluate their understanding. Just have them hold up (or in front of them) their number.
If you are a CI teacher, this could be used in combination with a signal for not understanding or going too fast. Like having students pass their hand over their head.