"I had no idea we were so unhappy"

So you're wondering what will warn you when your relationship is in trouble. Is there a distress flare to signal when you can ignore a relationship glitch and when it requires your undivided attention?
What ever you and your partner tangle about, it's the way you do your disagreeing that tells you how deep a hole your relationship is in.

American relationships researcher John Gottman can predict which couples will split up. He's right over 90% of the time. And it's not the topic of dispute that tips him off, it's the way you talk to each other that sets alarm bells going for him.

When you argue, what sort of a build up do you have? Going for the jugular early isn't a good sign. It stymies the chances of working anything out.  Probably your partner will get the idea that you don't believe this conversation will make a difference. And if you're not trying to make it work, why would they?

If your partner feels like you're getting at them that's not so good either. If you find fault or plain insult them, your partner will probably either shut you out or try to get even. Either way you're unlikely to get the response you want.

When you say what you don't like, keeping it specific helps. Focusing on the behaviour without getting personal is also a good idea.  "You didn't do the dishes last night" is a very different proposition from "you're a lazy slob and you never lift a finger to help around here".

You're human, so you probably do get at your partner now and then. The more you do it, the easier it becomes. Maybe you find it's escalating and next thing you know you're telling your partner they're a waste of space.  When that kind of scorn enters a relationship, it's definitely a sign of trouble.

When your partner sneers at you do you bristle and sneer right back ? Maybe you switch off instead and act like you don't notice and don't care. Refusing to engage is another indication that your relationship needs some serious attention.

So if it's warning signs you're after then research tells you that becoming hurtful quickly, really getting at each other, sneering, getting your back up and shutting down are the things to watch out for.
 
You might do all of these sometimes and not worry too much. What ever you fight about, if your typical pattern of fighting includes some of these then you can take them as warning flares.

The real question is what you do with the warning. The same research that identified the warning signs shows that what makes a relationship work is noticing and appreciating and building the good times you have together.

Warnings have their place. But if you really want to safeguard your relationship focus on what makes it worth having. You'll have a lot of fun plus you'll get a reason to bite your tongue before you sneer.

Don't wait for a warning before you attend to your relationship. Enjoy it right now.