Ideally you'd like to look for
(1) someone who is enthusiastic about learning
and
(2) can prove themselves capable of thinking for themselves
(also)
(3) they can pass a skill test
For (2): someone who can think up a solution to a problem, even if it's not the best solution, is infinitely more valuable than someone who either copies stuff off the web without understanding or someone who keeps bothering you for everything. At the same time, be careful they're not some arrogant prick (some students, may fall into this category: be careful that they are open to suggestions and don't scorn every idea that's not their own... I've seen this happen and it's not pleasant)
For (3): they will mostly have language knowledge if they're still studying and not much else; the test results will give you an idea about point (1): those who do well are enthusiastic about learning and understanding.. I'd do a test with difficult questions as well as easy/medium ones (stuff like trick C-pointers, function pointers), as that will tell you how they've learned what has been taught at school.