Love is an abstract noun. It is not easily defined, drawn, or explained. It can, however, be represented. Do you make each other laugh and smile? Do you fear for and feel responsible for the other persons safety? Do you enjoy the time you spend together? Do you build each other up and make each other better? If this person were not in your life, would your life be less complete?
You may have heard people say before how they cannot define or describe something, but they know it when they see it. Love falls into that category for me. In part because each loving relationship is different. The way in which you love one person may not be the way you love someone else. But I think it's safe to say that you'll know when you do genuinely love someone. It'll be undeniable and obvious.
I'm always less concerned with how love feels than how love acts. As far as I'm concerned, love is a commitment and a choice. That doesn't mean every commitment or choice is made out of love; however, if I've made that commitment the bad days or the hurtfulwords or the annoying idiosyncrasies will not dissuade me from that commitment. I've made my choice; and, unless something awful happens (such as abuse or other evils) I will stay committed. That's love.
I think you know it is really love when you begin thinking about how anything you might do will affect the other person. When you feel like you can not be without the other person for any reason.
All that is necessary is an observation and study of dogs. This is the only creature in the world who truly knows the meaning of love. Time and time again, the news reports how dogs rush to the rescue of their owners without a moment's thought of their own safety, how they place themselves in front of the owner and protect them from the cold, dying themselves of exposure.
How does the line of scripture go? "No greater love hath a man than that he lay down his life for another." A dog is willing to do this very thing 100% of the time. His life is in the eyes of his owner.
Now, while humans should not be slavish to the other whom they love, they must be willing to be unselfish and put the true needs of the other before their own desires. When a person considers what is best for the other before his/her own wants, when a person rejoices for another even though he/she suffers loss of some kind as a result of the other's gain, that person loves. Anyone who has owned and loved a dog [excluding the vicious, predatory types] understands this concept.
The simple answer, in my opinion, is that you cannot be sure. Ever.
My wife and I have been married for 20 years. We got married when were 20. Most people would say we were too young to have been getting married, but apparently we weren't. Even so, I still don't know how I was sure and I don't know that I'm sure now. After all, people do end up getting divorced after being married as long as we have been.
Look at it this way -- how can I know I'm in love with her? Is it because I can't imagine living without her? Plenty of people feel that way and then split up. Is it because I have lived with her this long? Same answer. Is it because she makes me feel complete? Same answer.
I have a hard time even defining love. And I have a harder time proving that I really am in love. That's because love is an emotion and there's no way to rationally know whether your emotions are "true."
I'm not sure if this is a prompt for an essay, or simply a personal question, but certainly, the responses to such a question are going to be as far ranging as the differences in personal experience with love.
I often begin my writing class with an easy (if cliche) definition prompt: "What is love?" I've heard at least 10 classes brainstorm answers to this question based on observations, experiences, and readings. Nearly always, the answer includes something about sacrifice. True love is willing to sacrifice. Some say true love is willing to sacrifice life for another, but others say the simple act of sacrificing long term dreams and goals, or a little bit of time every day, is the same thing.
From my personal experience, the best way to prove that a feeling is really love (and mutual) is to put it to the test of time. Too many people rush into relationships thinking if they don't act quickly they'll lose it. True love will wait forever. It is is a cliche, I know. But it is true.