1. Referrals by law enforcement
There are priceless sources of credibility out there if you contact the right people. Intelligence agencies, police and lawyers can be a good source of credible PI sources since they may sometimes employ these into their service. Another way to get that information is to find an association of private investigators, pretty much every state in the US has one, and there are others abroad if you are not from the Americas.
2. Make sure they are licensed
Although this license may vary on the country or state's local laws it is something you must absolutely make sure the person at least has that covered. Some laws require the investigators to have some prior experience in a similar field such as working with the police or intelligence agencies for a set number of hours. For the most part good PI's are found among retired members of these law enforcement agencies for obvious reasons.
3. Experience matters greatly
If there is one thing a PI must possess to succeed greatly at his/her job it is to have the necessary backing and experience provided by years in the field. Investigation is a demanding job, requiring careful sifting through information, traveling to various locations at times, the list goes on and on depending on the nature of the investigation. Also make sure there were no complaints against their
work and that they have a clean criminal record.
4. Be a good judge of character
When you meet the investigator you should trust your sense of them. Analyze and consider their character traits and whether they inspire confidence in their abilities. Stay away from people who give you “bad vibes” and overall a good sense of what they are. Although they are not held against a certain code of ethics or the type of confidentiality level a doctor-patient relationship has, PI know any information on their clients they give out is potentially threatening to their well-being. Speaking of which:
5. They are willing to testify in court
If they are truly professional and they know the workings of the legal system inside and out, they will know their testimonial carries great weight in court as an investigator. There are other factors you might consider of course like whether they have the necessary social qualities to stand in court and command the respect of the jury.