If you've written a good job post, and if they've responded with a thoughtful proposal, these core skills may have already been identified.

☛ Things to listen for: Understanding. Do they really understand your project? Core skills aren't necessarily technical skills, and it's possible a freelancer will list skills that aren't reflected in the description or the proposal. However, their response should show a firm understanding of your project or issues they may encounter.

☛ Follow-up question: "Of the three, which would you describe as your strongest skill? Your weakest?"