If you suspect your girlfriend is cheating on you, it's better to catch it early than to find out about it years down the road. Your girlfriend will often behave much differently than she used to if she's seeing someone else. Careful and objective observation can help you identify the signs a girl is cheating, but communication can also lead you to the same conclusion if approached the right way.
Look for signs that she's drifting away from the "together" aspect of the relationship and more towards being independent, such as: not including you in her life like she used to, refusal to talk about the future and doing more things on her own. She may be excluding you from her life consciously or subconsciously, but there has to be a reason for it.
Gauge her reaction when you give her compliments or decide not to join her for certain events, like an opera or musical. If she used to smile and pay attention to you when you gave her compliments before, but now suddenly she seems indifferent, then something is clearly wrong. Likewise, something is amiss if she doesn't care when you don't join her for events, or if she quickly finds someone else to go with her; or worse, if she doesn't ask you to come along.
Study any changes in her appearance. Your girlfriend probably always tried to look her best when you two first began dating, such as carefully applying makeup, always wearing beautiful clothes and working out. The urge to impress one another dies off after you've been together for a few months, so take note if she abruptly changes her style or always tries to appear her best. She may be trying to impress someone, but that someone may not be you.
Look for odd schedule patterns, such as coming home at odd hours, going out with her friends more often than she used to and constantly working overtime. If her schedule suddenly changes, there could be a valid reason why, such as a bigger workload or trying to catch up with friends. A schedule change in itself isn't a red flag, but it should raise questions if in conjunction with other signs that she's cheating.
Confront her about seeing another man. If the signs add up, then it's time to ask her why everything in your relationship has taken a turn for the worse and whether or not she's seeing another man. If she can give you concrete reasons as to why things have changed, take a look at trying to mend your relationship; but if she can't explain anything, then cheating is certainly a possible cause.