The best advice, in short answer form, is, Don’t Do It.
With all the new technology available today, it is sometimes very tempting to become your own detective in an attempt to collect damning information on an spouse whom you suspect of being up to no good. No need to hire a sleazy “private eye,” as in the old days, right?
For example, you may believe, since you own or co-own the family cars, that you are entitled to put a tracker on the car used by your spouse, unbeknownst to him or her. In Texas it is unlawful, a Class A misdemeanor, to install a tracking device on a car leased or owned by another person. Even if you technically “own” the car, if the other party uses it most of the time, you are on shaky ground, which has in some cases even lead to a grand jury investigation.
Putting spyware on your spouse’s cell phone can be a felony, even if you both use the phone and are both on the same phone plan. There are also tricky ways to copy your spouse’s email and have the copies sent to you, but again, you will be on very shaky ground legally. If the copied emails should include privileged communications between your spouse and his/her attorney, not only you but your own attorney (if you share the information) can be in very hot water.
But beyond the legal considerations, spying on your spouse, once exposed, will not make a good impression on a judge or jury. If you use the information that is revealed in the course of your divorce case, you will have to also reveal how you acquired this information. Maybe your spouse is doing something he/she should not be doing, but you may actually weaken your own case by appearing to be suspicious, devious and manipulative.