Once you have narrowed down your list of possibilities, contact them to determine if they specialise in your specific area of investigation or law. For an investigator to advise and choose the correct approach of an investigation, they need to understand the desired outcome of the client (i.e. matrimonial, criminal, civil, industrial) and the legalities relating to this.
Make sure you find out how long they have been doing the type of work you are asking them to do, and that your questions are answered to your complete satisfaction.
One thing that can separate investigators from one another is practical experience and the ability to quickly and efficiently acquire information through the use of informants and other means. Skilled private investigators are extremely adept at the artful task of gathering information that is either unknown or unavailable.
You need to ensure the investigator is someone you are comfortable talking to, and gives you clear rather than vague answers to your questions. You also have the right to ask if they or anyone employed by them has ever been arrested for a crime.
Also expect the investigator to ask many questions of you. Ethical investigators will ensure your request is genuine and that there will be no legal repercussions for performing the investigation.