Ask HN: How do you go about evaluating interns at the end of their contract?

What are some effective ways of evaluating interns at the end of their contract? I ask because we do not measure interns based on normal impact metrics as an employee. Interns are here to learn–and in turn help us learn by teaching–not affect our bottom line. So what are some good quantifiable ways to determine if they're a good fit to convert to a job offer?