Insurance and back up equipment
A lot of venues require an insured photographer on the day. It'll mean they take their job seriously and worry if their equipment fails or can't deliver on a job.
It's also best to check they have back up equipment, fight cases, etc. Having one camera and a couple of lenses isn't enough because things can go wrong. Make sure your photographer carries two pro bodies at all times to make sure those important moments aren't lost.
Keep to the above and you'll sleep soundly the night before confident that your day will be captured by a professional.