If you are afraid that the person you are with is cheating on you, then you need to really be sure before you make accusations. Sometimes it is easy to assume things and to fear the worst, often if you have insecurities, they might cause you to think the worst at times, but you need to make sure. At the same time, usually if you have suspicions or doubts, there is generally a reason and you want to make sure that you get to the bottom of it.
Resorting to snooping in your partner's things is probably not the best idea, and that is why the first step is to just talk things out rationally and ask them if there is anything they would like to tell you. You want to have this conversation in a calm manner, as it will often take on an accusatory tone that might push the other person away into a fit of rage, whether they are or not warranted accusations. That's why you really want to make sure you just put your fears and concerns on the table and see what the other person says. If he or she is cheating, they might come clean right then and there, but if not, then you need to listen to their explanation and then really trust your instinct.
If you are still not sure, then you need to discover where the source of your suspicion is coming from. This will take some self reflection and inner scrutiny. You will have to ask yourself why you think that person is cheating. Is it because he or she is acting different toward you? Do you feel them distant or strange? Did you catch them in a lie or some kind of fib? If that is the case, then you need to purse your suspicions further. But if you think that you have been feeling badly about yourself, maybe you have been performing poorly at work or you feel bad about your self image, then that could cause you to be projecting your personal feelings of inferiority out in your other aspects of life. You might assume that he or she is cheating because you just feel negative and in that case, it might be a case of misplaced suspicion.
But, if you really do think that something is off, and if your conversation has proved fruitless, then you need to figure out other means. First, try to keep some sort of tabs on the person's accounts. Lies always unravel and if you think that you are being lied to, keep track of the various excuses that you get, or the stories that do not match up and start to figure out how much of what your partner is saying is true. If something does not connect or you catch him or her in a lie, then it is time to resort to doing some investigating. That means you might want to take a look at phone records or through email messages to see if anything really is going on. And if your suspicions really are very intense, you might want to follow them around just once or twice at the times when you think something is going on.
But, be sure not to let this become an obsession or something that consumes all your time. It is important to know the truth, but once you know it, or once you are fairly sure, you need to let go. You have to confront the person about what you know and then you have to leave because you deserve much better.