The Student Room Group
Its actually quite obvious for the most part.

99% accurate is your gut. If you think they're cheating, you're almost certainly right.

Hiding a phone where it was once around.

Not as affectionate.

If you ask them about it, or anything related and they get really defensive, then they definately are.

There are more. I'll keep thinking.
Also:
Their moods may suddenly change for no reason.
They may suddenly get less affectionate for no apparent reason.
There may be changes in the way they communicate with you.
They may get unusually preoccupied, cagey, or defensive when you talk to them.
They may start mentioning a certain person a lot in conversation, when it isn't strictly relevant.
They can often become secretive.
They will very often become less interested in sex.
Reply 3
i agree with both above. Lack of sex, mentioning of names you havent heard before but the repeat of one, text messages, DEFINATELY 99% is gut feelings. Aggressive/defensive one minute, asks weird questions (my ex did this in the shower once like: 'hypethetically, what would you do if someone came on to you etc' but he was really stupid) Doesnt ring when they say they will, dont answer the phone when they are out.

The list is pretty long!
Reply 4
If something has happened with the other person, they'd probably be very uncomfortable when you try the same with them. I.e. if they kissed, and you kissed, there'd be some tension perhaps. When they are initiating the "affair" then they'd often try to find time to communicate, meet up, etc.

It's an incredibly difficult situation to bring up, as you run the risk of ruining the relationship whether its true or not - trust will always be a major factor, and perhaps only act upon it if you have really solid ground to be thinking that way. You could perhaps be honest and explain why you feel X is cheating, say the distance in your relationship, the secrecy, the defensiveness - gently ask if X is getting feelings for someone else.