Not all states have licensing requirements.To determine legitimacy ask about professional license, liability insurance, Better Business Membership, etc.
Have you worked this kind of investigation before?
If so, how many? Ask for examples. Be aware: if the investigator discloses confidential information regarding past cases can you trust him with your confidential information?\012
When does billing begin? Am I charged for travel time? Do you charge for mileage? Is there a minimum hours charge for surveillance work? Are there any other costs?\012