It's those that profess too much. They are convincing because they utter the same statement over and over, in an effort to convince themselves and you.
If 20 of us stood in a room having been told someone is cheating, could be one person or could be 19, but at least one person is not. You ask "ok which of you is a cheater" the chorus will be "not me" and all the while as they say it they are looking at you asking "are you cheating" it would breed suspicion. Even if you were told at the outset "only one person is cheating" you'd all be looking at each other, and all be feeling a little anxious that people might think it was you.
The truth is, almost everyone or no one could be cheating, and there is no way of knowing. It requires deception and lies. If it was open and unhidden it's not cheating at all. The cheating stops at the point of its discovery. It simply then becomes a fact that a person 'has' cheated. If you continue to have a relationship with them whilst they have a relationship with another, it's not cheating.
As far as I am aware no man I've been with has cheated on me, but I've not looked for it. DP has said he's never cheated on others. I have a feeling it's the one thing he hasn't been entirely honest about. When he's said this it was in response to a direct question very early in our relationship. He went quiet.
It requires deception, but mostly one has to decieve themselves. That's why you get the classic script when found out.
You can only ever point and say "he cheated" you can't look at someone and say "you're cheating" for all we know everyone else could be at it!