There are both benefits and drawbacks to hiring a planner who works alone, versus one who works with a larger team or company. If your planner works alone, determine whether he or she will be able to handle all the wedding details, especially on your wedding day. Ask the planner what happens if there's an emergency that keeps him or her from making it to the wedding -- is there someone qualified to fill in, or are you out of luck?
If the planner works as part of a team, find out whether you'll work directly with him or her or with a group of assistants. Make sure you understand exactly which tasks the planner is going to handle and what tasks are delegated to others (read: those with less experience). While a group effort can make for a successful wedding, it's best to find out ahead of time exactly who your point person is for certain tasks.