There is a psychologist (John Gottman) who can apparently predict whether couples will stay together with an 80% accuracy, after having spent 5 minutes with them. He refers to the four signs as being the four horsemen of the apocalypse:
Criticism - Instead of complaining about an issue in your relationship, you instead criticise the other person and make it their failing/fault.
Defensiveness - You refuse to accept responsibility for your own issues, and say it's the other person.
Stonewalling - 85% of the time, it's the man that does this. It is turning away from an issue, and refusing to discuss it, contemplate it, or resolve it. Instead, you blank the other person and act as if you couldn't care less what they say/do/feel.
Contempt - Abusing the other person through name-calling, sarcasm, and generally showing nothing but disgust for them. You have no interest in resolving the issues, you just want to hurt the other person. If you reach this stage, it is extremely unlikely that you will ever save your relationship.
Alternatively, she might be fucking another guy. Fucking Stacey.
TL;DR - There are 4 telltale signs to look out for, and you can fix them before they become too big of an issue if you look out for them.