• Ask whether preparing, planning and orchestrating the best ceremony possible is their primary concern?
• Is the reading of poetry, the organising and timing of music important to them?
• What recognised training have they completed?
• Ask whether they have a professional PA system? Especially if you have more than 30 guests.
• Ask how long before the wedding do they arrive, and when they go? Do they book weddings close together? How many weddings do they perform in a day or over a weekend?
• Ask do they supply resources for your to work with or other sources to assist you in your preparation (these are resoucres that would help in guiding you to make decisions on elements like vows, poetry, music etc).
• Ask do they dress to fit in with you wedding style and color scheme. What options on dress do they offer you?
• Ask whether they give a rehearsal on site (if they try to tell you this isn't necessary consider moving on to the next celebrant)
• Ask what you get in return for their fee - how many hours on average would they take preparing your ceremony.
• Ask how many meetings they offer.
  

Final WORD.
BEWARE OF CHEAP CELEBRANTS! Fees
Chances are they could be catering for the cheap end of the market, maybe doing too many, and not giving clients the time and attention needed. 
A guideline fee for a high class wedding celebrant at 2009 is $800-$1000.