One way to test for humility is have them do a programming challenge that isn't too difficult (it's not about whether they remember the algorithm for merge sort

One way to test for humility is have them do a programming challenge that isn't too difficult (it's not about whether they remember the algorithm for merge sorts), but has a few common pitfalls and arbitrary choices, and see how they react when you ask questions about their choices or make suggestions to them. Can they explain a choice? Can they follow a suggestion (or correctly and politely argue that it's unnecessary)?