One approach I use in class when trying to gauge readiness on a skill or concept is the consensogram. All that the students need is a pad of sticky notes (and a pen/pencil).
I put a quick graph on the board with 100% at the top and 0% at the bottom. Then I ask the students to write down on the sticky note how well they know the skill or concept. 100% means I could teach this skill to others while 0% means I have no idea what is being discussed at all (and name of the skill isn’t even familiar), and 70% means I know enough to show a basic understanding of the skill.
Once the students fill out the sticky notes and I collect them, I have a few kids place them on the drawn graph creating a bar graph. The first sticky note goes on the left, and the rest form the bar moving to the right.
This gives me and the class a quick snap shot of where the class thinks it is before a test, quiz, project, or before adding a new wrinkle to a skill.
Sometimes I have to reteach or firm up a skill while at other times we can move on without worry. Regardless, this allows the students to show me their confidence levels, and the students can see where everyone else is in relationship to themselves.
On some occasions I can get someone at the top levels of the graph to match up with someone near the middle (rarely is there ever anyone below 50%) of the graph. A little peer tutoring can sometimes work wonders, too.