There is a show that's been on TV a very long time called Cheaters.  I don't know how they've been able to do the same scenario for a dozen years, but they've pulled it off. 

I've seen the show about one and a half times.  Basically, someone who suspects their girlfriend or boyfriend of cheating hires this television program to do surveillance.  The crew follows the boyfriend or girlfriend, tracks their car, photographs them at various places (restaurants, stores, etc.), and tape records their conversations.  If he goes to a hotel, motel, or apartment, the cameras capture him going in, kissing his bimbo at the front door, and then grinding groins with her.  If a guy tells his wife, "Oh honey, I have to be at work late," the show will then cut to time-stamped footage of him going somewhere else. 

Near the end of each episode, the person who is being cheated on gets to see the tape, realizes they're right, and then feels very badly betrayed.  The program ends with the girl or guy confronting their husband/wife/boyfriend/girlfriend.  It's a free-for-all with the television cameras rolling.  The cheater and their bimbo or side stud start yelling, "What?!  What is this?," followed by a bunch of "beeps" (because of all the bad words they're saying).  I always think it's funny when the cheater says, "How could you do this to me?," in reference to being put under surveillance and exposed on TV.   Somehow they get moral outrage at that, not about the fact that they're screwing around on somebody.
 
Now, a lot of shows try to market merchandise to make money on the side: T-shirts, mugs, spaghetti sauce...whatever.  But for a show like Cheaters, it's a little different.  Since they can't exactly make shirts saying, "I'm a cheater," or, "I caught my spouse cheating," they have instead decided to open up a spy goods web store:

"The Cheater's Spy Shop sells all sorts of surveillance gear for suspicious minds, including recovery sticks that can pull up anything currently on the iPhone and even recover deleted information; mobile software that will send a person all texts and pictures being sent, web history, call logs, and GPS location every 30 minutes; and even motion-activated hidden cameras that record any movement and activity in high resolution [HD, 3D, whatever you want]...

The laws governing the use of surveillance devices by average citizens differ all over the place.  A lot of people think they're legally allowed to spy on their spouses, but depending on how it's done, that may or may not be true.  It may be totally legal to make, sell, and buy this stuff, but depending on the jurisdiction you're in, it may or may not be legal to use because people have privacy rights.  Some people also get concerned that stalkers could misuse the technology.  It's pretty scary to think about - a stalker could potentially put a tracking device at the bottom of your purse and know where you are at all times.  If you're thinking of participating in an operation to expose a cheater either with a private investigator or just by yourself, you have to make sure that whatever equipment or techniques you're using are legal in your state. 

"The two groups who seem to be buying the products the most are women worried their man is cheating, and parents who want to make sure their kids aren't sexting or getting inappropriate photos themselves."  The biggest month for buying is Valentine's Day.  They're not really sure why, but my guess is that women who either get nothing or get something worth less than what they see charged to their husband's credit card start wondering where the money went.

Cheating spouses typically get caught in a couple of ways:

1. Accidental discovery:  Most cheaters are not CIA agents.  They don't know how to totally cover their tracks, and they forget things like a parking sticker hanging from the rearview mirror.  Deception and infidelity are usually uncovered by somebody making a mistake.  "A husband or wife decides to come home from work early, a third party inadvertently reveals the truth, an unpaid parking ticket reveals a spouse's true whereabouts, or an e-mail exchange is accidentally sent to the wrong person."  Many times on the air, I've heard callers say, "He meant to send it to her, but somehow he clicked me."

2. Monitoring/Surveillance:  A lot of cheaters are exposed after being monitored by either their spouse or a private investigator.  From what I've read, if a private investigator uses a technique that's illegal, even if unbeknownst to you, you are still liable because they're essentially an extension of you.  Be careful!

So what should you do if you think your spouse or significant other is cheating?

If you're not just a hypersensitive or neurotic person, then your instinct that your spouse is cheating is probably right.  If you get suspicious, ask yourself the following question: "Why is he/she cheating?"  Did you make a mistake in picking someone who is simply a bad person?  If your wife had a million affairs while you were dating or your husband cheated while you were pregnant with your first kid (and then you went ahead and made three more) then "Duh!" - you made a mistake.  However, people don't always cheat because they're bad people.  Other things come into play, usually relating to the quality of the relationship.  As it turns out, men more than women require opposite-sex feedback for their egos.  Women can turn to their girlfriends to hear about what a bum their husband is and how wonderful they are.  But guys don't turn to their guy friends - they turn to other women. 

If you're a woman and are worried about your husband having an affair, you should read my book The Proper Care and Feeding of Husbands to see if you're to blame.  Your husband once picked you, wanted you, and ultimately loved you.  Perhaps if you stopped acting the way you currently are, you wouldn't be throwing away a perfectly good man.  A lot of times women call my show saying, "I think my husband's having an affair," and I tell them that they can either blow up the marriage or they can look at themselves honestly and admit, "I stopped being my husband's girlfriend.  I'm going to take the proper steps to turn it back on."   As his wife, you always have an edge over a new honey.  A new honey may be very exciting, but a new honey has no context or history like you have with him.  Losing you not only means losing the house, the kids, the dog, the parakeet and the cat, but he'll be losing extended family and friends as well.  There's so much for him to lose.  Missing the girlfriend doesn't come close to that.  So ultimately you have the power.

There are a million and a half articles on the topic of cheating, but it all boils down to one basic concept: if you treat your spouse and dearly beloved in such a way that he or she wants to come home to you every night, then you're doing great and you're probably not going to have a cheating spouse.

Now that being said, some people are simply jerks no matter how much love and effort you're putting in to the relationship.  The following article contains some practical tips on how to catch a cheater: "Tips for Discovering the Truth."  Discerning whether or not there is bad behavior going on (affairs, whores, drugs, etc.) usually helps you with securing custody of the children later.   And remember: Don't just ask, "Honey, are you having an affair?"  That never works.  Don't even bother.