If you're worried your boyfriend is cheating on you, your relationship's on thin ice, either because you don't trust him, or he really is cheating on you. Here are some things you can do to find out if your hunch is correct.

How to salvage the relationship

1. Realize that if it's gotten to the point that you seriously suspect he's cheating, your relationship is already on thin ice. 
    
2. Remember that honesty is the best policy. Talk about it. Don't try and catch him secretly. Just ask him up front. If he won't tell you the truth, then even catching him red-handed won't save your relationship. Ask him to explain his whereabouts or whatever else is troubling you. Don't take a simple yes or no answer. 
    
3. Remind him that you've entrusted him with your heart, and how much he means to you. Tell him how proud you are of him and your relationship - how strong and honest it is. If he gets uneasy or quiet, he might be hiding something, and the guilt will eventually push him to the edge. 
    
4. Tell him that you would never keep secrets from him, and mean it. Share with him a secret that you've kept from him; it might encourage him to admit any secrets he may be keeping, and will deepen your relationship.
     

Extreme measures

1. If you're serious about catching him and you're willing to breach his privacy, consider the following:
   • Spy software or monitoring programs may catch him in the act and get real evidence.
     For Mac OS X users: Install Perfect Keylogger for Mac
     For PC Windows users: Install Power Spy for Home
   • Get the password to his myspace or facebook account. If he's cheating on you he's probably talking to the other girl on myspace without you knowing about it.
   • Check his mobile phone for text messages from other girls. 
      
2. Listen to what he says. If he begins to point out the negative aspects of your relationship, that could be a sign that he is becoming or has already become unfaithful. Watch for comments like "Something is missing". 
    
3. Observe his actions. If he seems nervous for no reason or on edge, the odds are that he has done something he shouldn't have. He also may seem disconnected from you, or less fun to be around. 
    
4. Listen to what other people say. If he cheated on you, the odds are that someone knows. Gossip can be a very untrustworthy source - don't base your next move on this alone. 
    
5. If you're really this insecure about your boyfriend and you can't trust him, perhaps consider that you aren't ready for a relationship. If he loves you and you love him, you can work through any difficulties and there's no need for spying or hacking into his myspace/facebook/aim/msn, etc.
     

Tips

• Stay calm and collected throughout, even if you find out that he is cheating. 
   
• Remember that he was the one who blew it. Don't blame yourself for what he did. 
   
• Don't go after the girl who he cheated with. You can tell her off, but ultimately your boyfriend holds the moral responsibility. Don't resort to violence because that will just make you seem immature. 
   
• Has he accused you of cheating? Was it out of the blue and completely unrealistic? (This doesn't apply if you were being shady or acting suspiciously.) Sometimes he will accuse you of cheating because he's become paranoid from his actions and is projecting. 
   
• If he is being suspicious and seems to be hiding behaviors you may just want to consider breaking things off. Even if he isn't cheating he clearly doesn't respect you. 
   
• Don't smother him or try to monopolize his time to limit the chance of him cheating, he might be more likely to cheat if you do this. 
   
• Never accuse him of cheating. If you do he could clean up their behavior or start covering up more carefully. You won't be able to catch him if you give him warning. Just be observant. 
   

Warnings

• Don't make accusations until you gather the proper evidence. 
   
• Private investigators are expensive and probably not worth the money for non-marital relationship. 
   
• There is a risk if you follow him or invade his privacy. This may ruin the relationship even if he is not cheating. It is not right to spy on others or monitor their lives. 
   
• If you are in a long term adult relationship and financial properties are involved, you should consider consulting an investigator. 
   
• Invading his privacy without him knowing could potentially be illegal and totally immoral. You should NEVER attempt to do something that would monitor him unless he knows. 
   

Source: WikiHow.com