Don't ask if they can do a particular style (they'll all say yes), instead ask what kind of style they prefer to do. Use this tactic as much as you can so you get specific answers rather than allowing them to say what they think you want to hear.
What happens if it all goes wrong - if they're sick, backup gear, etc.?
Ask to see a variety of work, not just weddings (unless that's all they do). This will give a better idea of their style and general standard. Asking to see, for example, a complete album won't tell you much as you'll only see their best work.
Ask about their post processing work and to describe the setup, etc. so you can get a feel for how much post work they're doing. In my area at least most togs are just printing straight out of camera with maybe a de-saturation applied for B+W.
Ask for a demonstration of how they pose and relax people for the formals, etc. to see how they work with people. Will what their doing match the style of photography you're after? You won't get natural results if the tog is telling the groom how to hold the bride, etc.
How much planning will they do on a shooting list and schedule with the couple?
Lastly, I think the most important thing is are you on the same wavelength?
