You've likely already reviewed their portfolio and are familiar with what a freelancer has worked on - it's still important to have them walk you their history.

Pay attention to how they're describing them. What are they focusing on - results achieved? The team they worked with? The tasks at hand? Listen intently and notice what they get excited about when talking. You'll get a good sense of whether or not they'll be happy working in your industry or with your team, or with you!