The best content writer in the world is of no use to you and your business if they never submit their work to you. Some people might worry if a writer has a full-time job in addition to freelance-writing gigs, but if the writer is skilled at time management or realistic in what he or she can accomplish, a full-time job shouldn't be an issue.
Instead, ask writers about their process in completing jobs. Do they use the entire time available to them, or do they wait until the last few days to complete jobs. Have they ever turned work in late? If they have, did they let the client know ahead of time, or not?