First, make sure the investigator you hire is licensed in the state where their business is located. Some states do not require a private investigators' license. However, in California a license is required. I suggest prior to hiring a California private investigator, look up their license information at the State licensing bureau's website - BSIS. This site tells you how long the private investigator has been licensed, It also tells you whether there has been any discipline against the investigator.
The next step is to find a private investigator who is right for your needs. For example, if you have a situation where infidelity is the issue, then hire a surveillance specialist. Hiring an investigator who conducts bug sweeps and background investigations is not the right investigator for that situation.
Make sure the firm you hire (if found on the web) has a biography or "about us" section on their website. Many firms do not list the owner's name or anything about his or her background. I think it's very important to list biography information. It gives you an idea of what the investigator's background is all about and whether he or she is qualified to handle your case.
I believe it's important that the private investigator you hire is a member of a viable professional investigators' association. In California, the most well known, and largest association is the California Association of Licensed Investigators (CALI). It's also important that the private investigator you hire maintains continuing education. CALI has a Certified Professional Investigator program that requires 4 credits (24 hours) of continuing education every two years.
There are other questions people ask themselves before hiring a private investigator. The most common question is "Why can't I conduct my own investigation and save money?" There is an old saying, "He who represents himself has a fool for a client." This saying also applies to investigation. There are many reasons not to conduct your own detective work. Some examples are: You are bias when you do your own investigation; you cannot be objective; you cannot be an independent witness if you are called to testify. The most important reason not to conduct your own investigation is that you do not have the training or expertise to know what to look for, where to find the information and how to preserve the evidence or information.
I would be glad to answer any questions you may have about private investigation. Please do not hesitate to contact my office.