Talk to each student. Have them explain what they’re doing, what their challenges are, and what approach they are taking.
The good ones will know the surrounding lay of the land of their challenges, and may even be able to talk about multiple options and pros and cons.
The bad ones will “just mash it” or “try things until it works” or maybe “I know it when it feels right.”
Also, make a small project. Look at the solution. How does it perform during edge cases? What if you order -1 beers at the bar? What if you jump and hit your head on the underside of the moving platform? What if you set the end goal zone on fire? And so forth.
This is actually a lot of work for the teacher, which is why written assessments are so popular – the seem impartial, and they require minimal effort to prepare, give, and rate, and they lead to no discussion of partiality or unfairness. It’s just that they aren’t particularly good at assessing real skill.