Yes you will know because your heart will tell you how it feels about your relationship. You just have to listen to your heart. In many matters, mind is over heart, but in matters of heart, heart is over the head. Learn to listen to your heart, learn not to ignore your heart.
Regardless, you will know when your heart is not in the right place. Usually the politics of the situation will confuse you and whether openly or blindly you may follow the wrong path. But deep down in your heart, you will know something is wrong. When your thoughts wander mid day to dark places, or you are up in the middle of night wondering about something or nothing in particular, that is your heart telling you it's not happy. Unhappy heart won't let you rest. There is no peace of mind with restless heart. So listen to your heart mainly when it comes to your relationships.
Don't stay in an unhappy relationship for too long or your heart will break and your soul will be wounded. Don't be someone else's hero by sacrificing yourself and your heart. If someone is worth your efforts, he/she won't claim your soul or hurt your heart. Right relationships feels right, naturally. Don't get me wrong. True and mature relationships will have ups and downs and struggles and compromises. Those relationships will grow over time with you and closer to your heart. But after the first few quarrels and compromises, you will know if that relationship is a fair and long lasting one or not. In a fair relationship, you won't have to give all the time, you won't feel trapped and abused. Don't fool yourself under the name of ego, love, pride etc. If it's not the right relationship now, it never will be the right relationship, ever. So get out! Get out as soon as you figure it out. Claim your dignity, your inner peace and your content heart back by walking away from poisonous relationships. Listen carefully and your will know. Once you know, take action!!!!!