This is a question going through the minds of about 80% of people in a medium to long term relationship, and unfortunately 50% of the time the answer is yes!
Why is it that people feel the need to cheat on their partners? Well in my professional opinion the answers are virtually limitless. There are so many reasons why people have affairs that you could write a dozen books just on that topic.
What you want to know is how to spot that your partner is cheating on you, so here are some simple but very definite signs to look out for.

1) Actions will always speak louder than words. If your partner likes to talk, and talk, and talk, especially about your future and all the things they are going to do for you.. You need to block out their words and look at how they act, maybe read some body language books because this is a dead giveaway of someone who is cheating. They will try to talk themselves out of a hole, but their actions will always screw them.

2) There is never smoke without fire. This is almost always true unless there is someone out there who wants to hurt you. If there is a rumor that your partner is cheating on you, chances are it is true. These rumors are nearly always based on fact. You need to take a close look at your relationship and in particular at your partners actions if you hear a rumor like this.

3) Do their eyes wander? When you are out with your partner pay attention to what they are paying attention to. If you find that 9 times out of 10 they are not paying attention to you, but are eying up every attractive person in the vicinity then your warning bells should be ringing.

4) Know their friends. The majority of people especially women that I have caught cheating on their partners, have done so with the aid of a friend. This friend will almost always back up anything they say. For example if your partner tells you that they are meeting a friend for a drink after work, you will probably think this is fine, and even if you do call their friend they will often confirm what your partner has said. However most people find it harder to lie to a person they know and like, so get to know your partners friends and you will soon learn to tell when they are lying.

If you feel that your partner is cheating on you it can be a very traumatic experience, and can result in serious hardship both mentally and physically. The best thing to do in this situation is get in touch with a professional investigator who can quickly tell you if your fears are warranted.