Ask about their training-they should have taken courses related to this field.
Verify that they are full-time inspectors with significant experience (versus performing inspections to supplement other fields of business).
Ask for references. Then, call the references to see if they were satisfied.
Use only inspectors who will give you a written report of their findings.
Ask how much time the inspection will take and how much it will cost. A thorough inspection should last at least three hours, depending on the size of the property.
Ask about their limitation of liability agreement. This will let you know what they are and are not liable for if something was missed or an oversight occurred.