The categories below will guide you in creating your questions.
Consultant’s experience: Absolutely ask about her experience with your programs of interest, your demographic group, your industry. Yet understand that a really good consultant is always encountering new programs, changed programs, new types of applicants – and will excel with all of them. The consultant’s job is to go beyond the stats and the stereotypes to help you present your individual, distinctive qualifications and story. (See following point.) Also ask about her experience with various process aspects (e.g., will you need a lot of guidance in essay development, school selection?)
What’s not relevant? Questions about “success rates”– the consultant would need to have worked with a large population of applicants with your specific qualifications targeting your chosen programs (in their current state) for this data to be meaningful.
Consultant’s knowledge of programs: Ask the consultant how he keeps current and in-the-know about the programs.
The process: Before you talk to consultants, consider what services you need. Then ask the consultant how those services will be provided in practical terms – address time frames, information gathering, interviews/calls, editing process, who exactly you’ll be working with. In a nutshell, what he’ll do, when, and how.
Service differentiation: Ask what the consultant considers special and uniquely valuable about her service and expertise – you should receive a clear encapsulation of approach.
Accessibility: Ask about the consultant’s accessibility by email, phone/Skype – is she available on weekends, evenings?
Candor: Do you seek the benefit of the consultant’s full expertise? If so, if you target a program the consultant considers an unreasonably high reach or draft an essay the consultant considers off-target, will he tell you frankly?
Prices: Most consultants’ prices are listed on their web sites, but the services are often complex, so if you want clarification, ask. Make sure you understand if fees are time based ($X per hour) or flat rate ($Y for a defined basket of services).
AIGAC membership: The Association of International Graduate Admissions Consultants promotes professional standards and ethical guidelines for our profession. AIGAC members (individuals and firms) are vetted for acceptance into the organization, and adcom members at top b-schools attend annual AIGAC conferences. Ask prospective consultants if they are AIGAC members to confirm their professional qualification.