1) Other clients/references?
When it comes to corporate work ask the magician how many repeat clients they have. Any performer who is in high demand will have a lot of repeat clients. This tells you that the company was so thrilled with the quality of work the first time that they have booked the performer again and again. No performer should be reticent to provide references.
2) What is your pay schedule?
How much of a deposit does the magician require, and how much is refundable in the event of a cancellation? And should you wish to extend the performance(s) time, what would that cost? Is there a surcharge to bring additional equipment that may not be used?
3) What kind of preparation/dressing/facility will you require?
For some events a green room is required, and other events just a place to store my purse. It varies depending on the event and what is required from me.
4) If the magician you was is already booked will they recommend someone else?
A good performer will always recommend other people for your event. If they are unwilling to provide you with other options, then they might not has been the perfect performer for you. Do some research online investigating performers. It only takes a moment to look up other performers and do a little comparison shopping. A good performer will give you the information you need to help you make a decision, even if the decision is not to go with them. Clients want an event to be fun and memorable for their guests. Therefore I feel it is my job to have the guests talking about that party and the magician for months afterward.