What methods do private investigators use to determine who to investigate?

For the past 43 years, we haven't decided who to investigate, as that is almost always the client's decision. Granted, a PI may accept or decline a case based on several factors, but the initial matter is brought by a client, corporation, insurance company, or attorney.

Most top-echelon investigators have a checklist before they accept a retainer. The checklist includes personal safety, ethical considerations, legal guidelines, and client mental acuteness.