Ask questions that'll give you a 'feeling' as to what the job candidates may do in the future. What you're trying to do with your questions is weed-out those candidates you feel are just looking for additional skills and experience before venturing out on their own. You want to hire people interested in succeeding, but succeeding with you. Also, look for individuals who have a good track record of working for other companies. In other words, you could be looking for a needle in a haystack.
Good workers are so hard to find.
Once you do find them, you may want to consider having them sign a non-compete clause. Unless you're training someone to become a manager, whether or not to discuss business secrets can be a trying decision. If you feel that scenario is bothering the employee, then explain the fact that he has to work the first 5-6 hours of the day to cover the overhead before you make any money. This should bring him down to earth. If need be, lay it all out on paper for the employee to see.