The first warning sign is a failure to resolve the dispute. Second, it's a bad sign if there is a sense of helplessness. That leads to withdrawal, which heightens alienation. Finally, there's trouble if there are negative thoughts about the spouse. Negativity feeds on negativity.
Bad interactions lead to hostile perceptions, which lead to marital discontent.
They only see the bad side of the spouse, they give no credit for any good behavior. Happily married couples, on the other hand, tend to give their partner the benefit of the doubt when it comes to negative behavior. Couples who dredge up hurts from the past and escalate the hostilities are asking for trouble. It may seem cathartic to vent your spleen at your spouse, but it's flirting with danger. When husbands and wives disengage and avoid issues, the problem will only fester.