For us it's the tone of voice is often the first outward sign that there's trouble. Followed closely by not smiling at each other, rudeness justified as "I was just joking". For some reason when you're involved, you think you're fooling everyone and that no one knows - but the πkids know and other couples who've been there, recognize it and need to call it out. Pray with the couple in trouble, speak words of encouragement, give personal testimony of how God has helped you grow closer.