Do you even need a private investigator?
It would not be cost effective to hire a PI to accomplish tasks that you could do for yourself. Accomplish all you can on your own. If you then determine that you need a private investigator, provide him/her with as much information as possible. This saves the PI time and you money.
What type of investigator do you need?
Some investigators specialize in specific areas and others provide many types of services. Pick a PI that specializes in your need or has a strong background in that area. Be skeptical of those who say they can do anything and advertise a multitude of services.
What to look for.
Ensure that the PI has a license in the state where he/she operates, if required. Also ensure that the PI is either insured or bonded. Check out his/her background, training and experience in general and in the specific service that you require. Ask for references. Experience with federal or state agencies as a sworn investigator is not a necessity, but is an excellent reference. Check out the professional organizations to which the PI belongs.
What to expect.
PIs can usually accomplish much more than the average person due to their experience in the field and their contacts. However, every case is different and results vary greatly. Even the best investigators come up empty on occasion. If an investigator guarentees results, be wary. You should provide the PI with clear instructions on what you wish to accomplish. He/she in turn should give you an estimation of the time and expense involved and the probability of success. You should be kept informed on a regular basis as to the status of the investigation. Any work necessary that is in addition to the original request must be approved by you. Most investigators require a substantial retainer up front. Ensure that you understand the fee and expense structure. Houly fees vary from $50 to $250 per hour depending on location and type of service. In addition, mileage and actual expenses are also charged.