If you really want to know if someone has the capacity to pull their weight as an engineer, ask them about what they've built. Even if they are fresh out of college, the best engineers will have projects they can talk about and explain. Ask how they approached/solved specific problems. Ask what they're most proud of building. Ask what was most frustrating.
Those are the kind of questions that will provide insight into a person's problem solving capabilities and offer a decent picture of what they're capable of doing.
Now that I know this would be for a functional language, it makes way more sense. At the time I couldn't get over why you wouldn't just use a doubly-linked list like the standard implementation in most OO languages.
"To find out if they can get stuff done, I just ask what they’ve done. If someone can actually get stuff done they should have done so by now. It’s hard to be a good programmer without some previous experience and these days anyone can get some experience by starting or contributing to a free software project."