In lieu of GPA I ask what kind of classes they took and what were their favorites. That list is a much better indicator, I think. It also lets you gauge what kinds of questions you can reasonably ask and the types of solutions that you can expect. If someone didn't have to study graph theory, they likely won't see a graph theoretic answer and you shouldn't as a question that leads heavily to that type of solution, and if someone studied a lot of combinatorics, you might see them bijecting your problem to other problems they've seen.