Obviously it's then important to then check understanding.
"How do you know ....?" "What do you need us to do to help you improve?"
I have a portable laminated class set of these and then students also have them stuck inside the front of their exercise books too.
The good thing about them is that learners can indicate more than one position at once - where they started the lesson with one finger and where are they now with another as a simple way for them to see that they have made progress.
Copies can be downloaded here from TES.