It's relatively common in divorces for one spouse to look through the other's private property, especially when infidelity is suspected. While it may seem that there is little that can be considered private property in the context of a marriage relationship, a legal distinction can be made.
For example, if you break into your spouse's e-mail account to read messages, you may be crossing a line that will create a serious problem for you. Recent court developments outside New York suggest it could potentially be considered a crime here.
The safest approach to gathering any evidence, whether you suspect adultery or something else, is to first consult a knowledgeable matrimonial attorney. You can discuss the option of hiring a private investigator, trained in legal ways of compiling proof of your claims, so they will be admissible in court.
Aside from the legal implications, being a snooper does not to give a positive impression to the judge who is ruling on your case.