The Discovery Process
Discovery is just what it sounds like: You discover what information your spouse possesses and what she might possibly present at trial. The only real limitation is that the information sought must be pertinent to your proceedings. Various methods are commonly used to get this information such as subpoenas, interrogatories, requests for documents and depositions. Subpoenas are issued to third parties -- such as private investigators -- who may possess documents or information relating to your case. A request for documents demands that your spouse turn over information that she has in her possession, such as documents a P.I. already turned over to her.
Other Disclosure Requirements
If you're not aware that your spouse has hired a private investigator, this prevents you from asking for the documents as part of the discovery process. She has a legal obligation to provide you with information anyway. She usually can't spring the documents on you at trial without warning. If the investigator has come up with sensitive information -- or even information that's not sensitive -- she must serve you with a copy of the investigator's reports. This is true even if she decides not to use them at trial for some reason, such as because they incriminate her as well.
Issues of Timing
Your soon-to-be ex must serve you with copies of all information that she may use at trial within a statutory period of time. For example, in Arizona, she must provide you with her evidence at least 60 days in advance. If she doesn't, the judge may not allow her to present the evidence, ruling that it’s inadmissible. An exception to this rule exists if the investigator's documents pertain to custody matters. In this instance, the welfare of your children usually trumps considerations of timeliness, so the judge can consider the information even if you haven't previously seen it. If she gets away with surprising you and leaving you scrambling to defend yourself, the judge can impose sanctions on her for breaking the rules of discovery, such as fines or other punishment, even if he doesn't preclude the information from consideration.
Investigators Can Testify
Third parties such as private investigators can -- and often do -- testify at divorce trials, so it's possible that your spouse or her lawyer may not serve you with written documents ahead of time. They'll simply present the private investigator as a witness. Your spouse must still give you warning, however. Most states require that spouses exchange witness lists before trial, disclosing the persons they intend to call to testify, and about what each individual is going to testify. If you find an investigator's name on her witness list, you can depose him ahead of time. This involves meeting with him and asking him questions in the presence of a court reporter who takes down every word. The court reporter will provide you with a transcript of the dialogue, which you can then present as evidence at trial if he says something that's beneficial to your case. You or your attorney will also get an idea of what questions to ask him at trial when you have the opportunity to cross-examine him.