Every relationship can have it's struggles. Sometimes it's just small things, like arguing over dinner, or what to watch on tv. Sometimes there's serious signs, that are showing the relationship is ending though. Not every serious problem means the relationship is dead.
Make sure you aren't being premature with these decisions. It's always important to talk about a problem you and your loved one may be having.
See if communication is a problem. If you two aren't communicating to one another anymore. If the communication has stopped, then this is a huge sign that it's coming to an end. I don't mean talking to each other, but sharing to one another your problems and successes. Opening up to each other is key to any relationship. If you two are keeping things from each other, that's one of the biggest red flags. If it's gotten to the point, where you and your partner can longer open up. Then the relationships is dead.
See if you are still trusting them. Obviously trust is everything, but if you really can no longer trust them it's over. If they can no longer trust you, it's not a healthy relationship. You can't continue to live your life always doubting their word, or actions, and expect to be happy. You can't always feel they are doing the same to you. There's no progress being made.
See whether or not you can get over past mistakes. If you yourself can no longer learn from the mistakes you've made, or the same goes with your partner. You just continue to repeat the problems you've had without making progress at all. This repeating cycle is not healthy and speaks loudly you've reached your peak in the relationship. It's no longer growing, it's just deteriorating.
See if you are always angry and jealous with them. If they continue to disappoint you, and you can no longer remember the good parts you love about them. Basically if the bad is just out weighing the good. Make a list of the good times, or things about them. Then make the list of the bad. Ask yourself if this is really making you happy. Is this what is best for the both of you?
Getting them to open up physically is difficult. No more sex in the relationship. If you and your partner are no longer opening up physically with one another, then the flame and passion has possibly gone away. Sometimes there's more to this, like a physical problem. So be sure to know what it is first.
See if they are not willing to improve. If they aren't willing to try to work past things, then that's the ultimate sign it's time to move on. Any relationship must grow. If their actions continue to hurt you and they don't want to improve from this. They keep promising never again, but they keep doing the opposite. They are giving you false hopes and it's time to move on.
Do you find yourself wanting your partner to change? If you figure that you want your partner to change more, than actually being who he or she is. This is the ultimate sign, he or she just wasn't what you thought you were getting when you first married, or dated them. False hopes have become a reality.
Make sure they're not abusive. If your partner is becoming mentally or physically abusive to you. Obviously this is a sign to get out of the relationship. Anyone who says they'd never do it again, is full of it. Get out right away and hope they get the help needed.
See if they are starting to forget about you and if they're starting to be around you much less. If they are going down the path, where they are always working, not remembering anniversaries and birthdays. Basically they become so consumed in everything but you. Don't honestly end it, if they happen to do this once or twice. Sometimes they just might be under a bit of stress. If this becomes the normal, routine kind of thing. No communication on your part can get through to them about this. This is not a healthy, loving relationship anymore.