Number five on the Four Foundations Top Ten list is:
"I make it a habit to be sure that formative assessments are involuntary and all-inclusive."
This habit was mentioned in the previous post, but it's so important that a review is in order.
Checking for understanding should occur frequently during the learning, and we need to plan to check all students and not only the eager volunteers.
In fact, I'll go out on a limb and say that we should rarely call on students with their hands up. We should check the entire class often, and when we do call on individuals it should be with targeted cold calls rather than responding to volunteers. We may want to take volunteers occasionally as a way to encourage students, but normal procedure for checking for understanding should be involuntary and 'all-play'.
There's more on formative assessment in Four Foundations of Great Teaching (pp 10-16).