Whether or not a private investigator finds proof of a problem you thought existed could be good or bad, depending on what you had them look for. Regardless, if they do find something, then you need to know what to do next. You would think the next step would be obvious — call a lawyer, call the police, and so on — but there are a few extra steps you should take. These may seem very small or unnecessary, but in the end, they can save you a lot of trouble.
Make Sure a Twin Wasn't Involved
This is not a joke. While it sounds like the stuff of dime store novels from the 1940s, sometimes you think you catch someone in the act of doing something, and it turns out to be a twin. This is especially a problem when you've hired the private investigator to look for insurance fraud, like someone claiming they were injured, and then another person sees them playing a sport that they shouldn't be able to play while injured. It sounds unrealistic, but it is real.
It is possible, too, that there's still something funny going on, and the twin is involved. But this is something you and the private investigator need to clear up before making any accusations.
Make Sure the Private Investigator's Evidence Is All Legal
A good private investigator does everything possible to ensure evidence is collected through legal means only. Unfortunately, technicalities can trip up even the best investigators. It always helps to review everything to make sure there are no loopholes or issues that could be misunderstood that could make the evidence inadmissible in court.
Make Sure Your Expectations Are Realistic
The private investigator can do only so much. They can't officially arrest people, for example; they can do citizen's arrests in states that allow those, but they may decide at the time that the arrest could make things worse or potentially provide the suspect with an opportunity to accuse the investigator of illegal tactics. There may also be other issues where the investigator couldn't get all of the information you wanted and says it's not possible to try because there's no way to legally obtain it. In this case, do not demand the investigator go out and do what they can to get that information. That is not going to help you.
A private investigator is a helpful person to work with when you need evidence that something is or isn't happening, and it's tempting to let all emotion loose when the investigator finds something. Take time instead to ensure that all of the evidence you now have is evidence you can use.