Please understand the content of this page is for reference use only. Do educate yourself about the legal ramifications of any action prior to engaging in such activity.
In an ideal world none of the content here would be necessary. Open communication should be the primary source of gathering information. A licensed marriage counselor can help facilitate the conversation and build a transparency between you and your spouse. This should be your first (and hopefully last) stop on this page; Marriage Counseling.
PIs are expensive and deliver results when the spouse is caught in the act. I'm referring to surveillance for the PI activity here - computer stuff is below. Surveilaance is the oldest and most often recommended service. This isn't practical for a number of reasons. First, when a betrayed spouse believes his partner is cheating there is an immediate need to know. The cheating partner often won't have a conveniently timed rendezvous. Second, the cost for tailing someone is prohibitively high. Finally, although there are exceptions, the associates most often sent to stake out are junior members of the firm or independent contractors. The quality of the surveillance is often questionable, following the adage that you get what you pay for (cheap labor generally delivers less reliable results).
PIs and lawyers often recommend computer forensics to delve into the behaviors of the wayward spouse. Given the amount of information that is digital these days it is a good place to dig for the truth. However, this is an expensive prospect. I was quoted at $800 for a computer and $750 for a smartphone. I tried them out on the smartphone and I received less than stellar results from a firm with a very good reputation. I decided to find out what I could on my own. Many posts on this blog will detail the tools I used. Please remember I'm not a computer programmer or forensics guy... Just a moderately computer literate schlep with a burning desire to get to the truth.
Nota Bene: That means pay attention to this- if you do your own investigative work, it may not be admissible in court. Talk to a lawyer.