How do you know when you are "in love"? Certainly it is different for everyone. But the short answer to your question is you really don't know --- not at first. There is no "love at first sight." That crock of shit is based on the biological and energetic attraction. The "new relationship energy" (NRE) in the beginning of a relationship could also sustain the 'love at first sight' claim. But NRE feelings are levels of infatuation and lust all wrapped up in the newness of it all and often impetuously referred to as "love".
Claiming that you can't live without someone is a bunch of bullshit too, otherwise you would have been dead before you met your oxygen bottle. Never underestimate codependent tendencies, abandonment issues, and other such fear issues that ripen in an otherwise secure relationship.
Oddly, deep resounding love that knows no restriction, no intolerance, no jealousy, no fear, and will never end (may change over time but never end), is seldom recognizable at first. The overwhelming power of NRE is a blinding light that may or may not obscure the beginning of deep resounding love.
But after some time (different for everyone) you get it. You experience the presence of your loved one as though they were and extension of yourself. You know that your love for them will continue, even if they leave you. The deep love you feel, may be overshadowed for a while by anger, but the love never leaves you. It has changed you from who you were before, to what you are during, and until you die. And this quality of love can not be measured or determined at any given time. It just is.
