Request a list of client references. Request a list of suppliers that have been used in the last year. From these you should inquire about the professionalism of your candidates. Discuss concerns about support. Expect sound planning to be evident in their reply.
Some good companies contract out for help when needed.
Discussing perceived short falls openly and with a positive action plan to address your concerns is reflective of a viable planner. If the following questions are not part of your conversation, find a more knowledgeable partner. These questions are basic indicators of an experienced planner. Be suspect of planners who say they can do everything. When considering placing a meeting or event with a planner, listen for recommendations by the planner to bring in experts, such as destination management companies, or other professionals.