Have them compare their solution to the coding exercise with another solution. It could be the solution from another pair in the same group, or from a previous group tackling the same problem, or one you came up with. Give them a checklist of things specifically to look at, which you designed with this exercise in mind. For example things like:
Remind them to be kind and respectful of the authors of the code they are comparing with. Even if those authors are not present.
Based on what they learnt in the code comparison, ask them to make a shortlist of things to remember next time they use this technique/tackle this kind of problem.
They might get an idea of how far they have to go before they master the technique. If they share their things to remember checklist with you, remind them to use it next time you tackle a similar problem.