If you suspect that your boyfriend or girlfriend is cheating on you, then you need to take action now. The longer a relationship continues with this distrust between you, the greater it will grow.
That said, these three signs will help you to determine whether you're being paranoid, or you have legitimate cause for concern:
A Sudden Change in the Hours They Keep
If your partner is all of a sudden coming home three hours later than usual, it might just mean that they're taking extra hours on at work. If they don't want you to come by at work to see them, then that's probably not what it means.
Extensive Business Trips
The "Business Trip" is the excuse that our parents' generation used to create an alibi. It might not be so simple, it could be any excuse that is simply too convenient and allows them to be out of the house alone for days on end. Again, this isn't always a sure sign that someone is cheating, but you shouldn't be ashamed if extensive trips alone get you suspicious.
Sometimes the sign isn't too clear; sometimes it's simply your intuition telling you that things aren't quite right. You know your partner and your relationship better than anybody, you know the signs, and sometimes something triggers your subconscious and lets you know that not all is on the up and up. It might not be an affair, but it might be worth looking into nonetheless.
Trust is a hard-earned thing in a relationship. When you cannot trust your partner, you cannot live with them. It's better to put your mind at ease than it is to carry doubts with you through a whole relationship.
With that in mind, one of the easiest ways to avoid causing a rift is to talk with your partner, first. Even if these signs don't point to cheating, they may point to dissatisfaction in one or both partners. If your relationship is important enough to you that you're worried that they're cheating on you in the first place, then it should be important enough to talk things out before making accusations or trying to investigate.