When two people no longer want the same things or aren't on the same page the relationship is pretty much over. Either one person works up the courage to call it off or they start a new relationship/cheat. Going along to get along never leads to happiness. You don't have to wait for someone else to end a relationship you are not happy with.
If you are unhappy with the way things are then you communicate it to your significant other. If they are happy with the ways things are then it's time for you to move on. The warning signs are your instincts or you feelings about the relationship. The people who don't see "the signs" are those who choose to ignore their feelings or instincts. My take on it is, "If it doesn't feel right then it's probably not right for you." Many people can't walk away from a relationship unless there is cheating, abuse, or some type of drama. It's as if being "unhappy" or not wanting the same things for the relationship are not enough reasons for them to walk away.