So you’re pretty sure your partner is cheating. Okay, you’re convinced. But don’t say anything! Don’t confront him, don’t ask the friends or family, and above all–do not accuse him! Now is the time for you to become your own private investigator and get the evidence. Actually, hiring a licensed private investigator is the best way to go if you can afford it. That way, you know everything is being done legally and thoroughly.

WARNING: Before using ANY methods of gathering evidence against your suspected cheater, you need to know that in Michigan, a man has been charged with a FELONY for reading his cheating wife’s email! The statute covers theft of intellectual property. If convicted, the man could face up to five years in prison! Be sure to check the laws in your state before using ANY means of getting the evidence!
First and foremost–do not get caught! Easy for me to say, right? I’m just saying that I know you’re anxious, upset, angry, and myriad other feelings that could interfere with your ability to be cool. If you get caught spying, then you end up looking like the bad guy if he isn’t guilty. And if he is guilty, you give him the heads up that you’re onto him and that even more care should be taken not to get caught, which makes getting the evidence even more difficult for you.
I’d say the easiest thing for you to do would be to look at the phones he uses. Unless he is stupid, he will probably delete calls from the caller ID and calls on the cell, as well as texts (or sexts!). However, if you’ve been really cool about the whole thing, he might be overly confident that he has your complete trust and would never suspect a thing. (I actually heard a guy say that before–that his wife would never suspect he would cheat and would never check up on him. Turns out, he was right!) 

The next easiest thing is to check his computer. Check history and look for suspicious sites such as dating sites. Most cheaters are smart enough to set their history to 0 days, but again, they might think they have your undying trust. If you know (or can figure out) his passwords, look at the email. They don’t like to delete juicy emails! And don’t forget to look at the pictures! Okay, a bit of a warning here–some guys are kinda pathetic and save pictures of women they don’t even know that  they found somewhere online. Just because it’s a sexy pic doesn’t necessarily mean it’s someone he’s seeing. But if it’s a picture of someone he knows… that’s a different story!
The next time he says he’s going out with friends, ask where they’re going to be and say, “I might stop by,” or “I (or we) might join you for a while.” He might say, “I don’t know. I’ll call you when we get there.” If you don’t get the call, you know your suspicions are probably right. If he is evasive, saying, “Oh, I’m not sure. We’ll probably hit several places,” he doesn’t want you there. Ask yourself why.  
If he always goes to the same place, or he tells you where they will be, go there. Don’t go in looking like Inspector Clouseau, hiding behind ficus trees and that sort of nonsense. Just walk in like anyone else and scope the place out. If you see him talking to a woman, DO NOT PANIC! Even if they look cozy! Be cool. If you’re out of their line of sight, watch. Body language says everything, especially when there is alcohol involved.  

Compose yourself, stop shaking, and calmly walk over with a smile. Introduce yourself to the woman as the wife (or girlfriend) of the suspected cheater. Watch for a reaction in both parties. If the woman looks shocked or surprised, or especially if she looks angry, you know your partner didn’t tell her about you. If your partner looks annoyed or angry that you’re there, you can be pretty sure he is either cheating or looking to cheat.
There are other ways to catch an unfaithful partner, of which I’m sure you’ve heard in the media. We all know about recorders you can buy to record calls on your landline. One model I saw was $299.95, but you could add the optional truth detection/deception software for an additional $100.00! I didn’t know about the truth detection/deception software! 
Be careful in recording phone conversations. In some states, it’s illegal to record a phone conversation unless both parties are aware that the conversation is being recorded. They either have to be warned that the call is being recorded, or the recorder has to give a signal that clicks or beeps intermittently, letting them know the phone could be tapped (which, of course, defeats the whole purpose!). You could do this for your own information, but if you let the cheater know you did it, you could be prosecuted. And whatever you recorded wouldn’t be admissible in court, because it was obtained illegally.
You’ve probably also heard about computer monitoring software you can install on your computer that records every keystroke. I saw one you could download for only $97.00, and it claimed to be “invisible” (undetectable).  
What you might not know about is that there is also software which can detect the computer monitoring software you could install on your partner’s computer. It lets him know there is spy software on the computer, in which case you’re busted. At the same time, you have to remind yourself that if he has put that type of software on his computer, he has something to hide. As Dr. Phil (McGraw) says, “Those who have nothing to hide, hide nothing!” 
If you think your partner might be suspicious that you’re suspicious, don’t use the spy software. You can get spy hardware for your computer. It can’t be detected by the software he might install to catch you snooping. One model is priced from $89.00 to $299.00.
You probably also know about the “nanny cams” that you can set up to catch a cheating partner. “Cheater cams” are motion-activated hidden cameras, and some even work in the dark, but be prepared to pay a pretty penny for those! One model I saw listed a sale price of $499.95! Some of them even look like smoke detectors or clothing hooks–pretty stealthy!
You’re probably also aware of the GPS trackers you can buy to put on his vehicle, which would let you know where he is. I saw one online for $369.00.
The next product is one of which I wasn’t aware. There is software you can put on a cell phone which tells you everything–you can view contact lists, read text messages, view calls made and received, and even view pictures! The price of this model was listed as $138.00, but was said to be going up to $199.00 soon. I haven’t heard of any laws about this one, but you’d better check before buying it.
Get ready, people, because I saved the best for last! I was shocked at this one. Are you sitting down? It’s a semen detector! It was marketed for women to find semen in a man’s underwear, but how much more damning and convicting would it be to find semen in a woman’s undies? (Remember Monica’s blue dress?!) This was priced at $49.95. 
I’m not sure which information obtained by these means (if any) would be admissible in court. You might be able to find out online under a search for divorce laws in your particular state. If you couldn’t find out that way, you could ask a private investigator or attorney.
If you can afford to, hire a private investigator to do all (or at least, most) of the work for you. There are many ways that you can catch a cheater, so before you confront him, get the evidence! 
None of the prices listed for products available to catch cheaters include shipping and processing charges.