they are less attentive, don't want sex , forgets special days, is always late for things. they say something that suggest it. normally your gut will tell you. most times you just don't want believe it and try ito gnore your feeling.
i remember crying a lot even before i knew, checking for lipsticks or even letters...
Their fingerprints are on that knife sticking in your back.
Without hard proof/evidence, you can't. However, generally, if something doesn't feel or seem right, it usually isn't. Intuition is our internal "alarm" that often tells us if something is wrong or right.
The realization that you have been betrayed is always discovered, too trusting you pay the price.
Betrayal closes all doors to trust. But it also makes us more wary lessons can be learned.
look at the actions of the person
if they act differently around you, and by the things they say or do. body language plays a big part.