Through thick and thin

A person may be able to know they are truly in love because the other person in the relationship has stayed with them through the good times and the bad. And in turn they have stayed with their boyfriend or girlfriend through their ups and downs as well. Many people have relationships where the other person leaves (or they leave) at the first sign of trouble. But when they have a relationship where they don't feel like leaving and they feel like helping the person through it, or working through the problems, they may be in love.

Sacrifice

Along with going through the ups and downs and hard times with the person, a person may know they are in love if they are willing to really sacrifice for the other person. When a person is willing to be selfless and think more about another person than they do themselves they are likely in love.

Long lasting

Sometimes a person may know they are in love because the relationship has lasted for a long time. Perhaps their past relationships have ended sooner or have not been able to last. When a person finds a person that they are happy to be with for a long time they may be in love with that person.

Of course the length of the relationship is not the only factor in determining if two people are really in love. There are several other reasons why a relationship may be long lasting. For example, sometimes two people are afraid to be alone or worry that breaking up is too hard and so they prefer to stay together in a loveless relationship.

Time

Some people know they are in love because they are able to talk to their boyfriend or girlfriend for hours on end without running out of things to talk about. They are able to spend time with them doing anything without getting sick of the other person. They understand that they may have to be apart because of work, school, etc., but can't wait to spend time with each other. And the activities they do together do not need to be exciting and thrilling activities, they can be simple things. The important thing is just to be together.

You just feel it

A lot of people know they are in love because they just feel it. Usually a person who just feels like they are in love can compare their current "in love" feelings with previous feelings of infatuation or strong liking. They are able to feel the difference between these feelings. Love is stronger than infatuation and liking and if a person has felt the different feelings they are more likely to know the difference.

Past initial attraction

Many people confuse feelings of attraction, infatuation, or liking for love. This confusion usually occurs in the first few weeks or month of a relationship when everything is new and wonderful. As these feeling wear off if a person still feels a strong feeling for the other person it is very likely they love the person. They should still be attracted and like the person as well, but for someone to truly be in love those other feelings will take a backseat to love.

Sharing emotions

Many people feel they are in love when they are happy when their partner is happy or sad when their partner is sad. They want their partner to succeed and do well in life and support them in all that they do.