A (major change in behavior) or treatment towards you is the key.
Just as Zoenchi noted distance/spending less time together, secretive, less contact throughout the day, not returning calls in a timely manner or being less available, seldom initiating contact, dramatic drop in affection/sex, the absence of eye contact during meals and conversations,becoming easily irritated or angry over the slightest things, reacts defensively whenever you ask where they were by accusing you of being insecure or having trust issues while never answering the question, a sudden interest in working out and buying new clothes, showering you with gifts (after being absent) for stretches of time, rarely saying "I love you", and a general feeling that they are simply "going through the motions".
Once again it really comes down to them no longer behaving like themselves. If something doesn't feel right to you then it's probably not right for you. Unless you have a history of paranoid and insecure then you should simply trust your instincts and move on.
Know yourself, Love yourself, Trust yourself
Ignoring "red flags" can lead to both regret and heartache
The bottom line is once you (stop trusting your mate) the relationship is doomed whether you're right or wrong.