You never know if you are in love forever until you are. Which is to say, you die and you are still in love.
There is no way to compare your love to anyone else’s. There is no test for being in love. It is something you decide for yourself. It is a feeling or an intuition.
Poets have written about love for millenniums. There are so many stories. Psychologists have also written about it at length. People will tell you that love is this or love is that. It may or may not be true for you.
What @marinelife said is a good guideline, but by no means the definitive guideline. Love can be the feeling you have when you are around someone or communicating with them. Love can be a desire to do things together. Love can be how you feel when you are near someone.
Love can be expressed in many ways, too. Love has a physical component. It has a relationship component. It has a communication component and a creative component. When you’re in love you might want to be creative with the person—marry them and make new life together and nurture that life.
But when I am in love, the best part for me is that overwhelming desire to be with her and to please her, and to do everything I can for her happiness. I look forward to every contact with her. I can’t wait until the next one. She is always there in the back of my mind. There’s more, of course, but I don’t want to spend days writing this.
So that’s what it’s like for me. It might be quite different for you.