Do you know about Love?

So a very good question was posed to me today - "How do you know when you really love someone?" I am sure it is something we have all thought about at some point in time, and I think the answer is probably based on what point of your own life you are in. By that I mean if you have kids, you base it on unconditional love; if you are in a good relationship or a bad, you base it on that; if you are speaking to family, you base it on the obvious - you just do.
But then I thought is that really a sufficient answer? The most frequent answer people give is you just "know". Well if someone is asking you "how do you know" then they apparently it is not that easy for them. And I would say it is not fair to give them that answer. Some people might not know; there are times I thought I knew and in hindsight, I didn't.
So is that the answer? Do you only know in hindsight how much you loved someone? Do you not know til they are gone or you have something better? Now remember this can apply to not only relationships but also family, friends. There are different kinds of love so I suppose the answer would vary depending on their frame of reference. Is it possible then that no one really knows? How can you clearly define that when so much goes into the answer - e.g. emotions, other parts of life like work and money, the object of your "love" and their behavior.
So, my question is - what is your answer on this? If someone asked you that, what would you say? Thoughts thoughts?! Comments please......