'How do I know I am in Love? And not in Lust, or in Longing? Although all these can also life comfortably(?) with Love.
In some ways, at least in my experience, a quick answer is - If you have to ask, it's not love. I think Louis Armstrong meant much the same when asked 'what is Jazz' - 'If you have to ask' came the reply, 'you ain't never gonna know' or something like that.
I believe there is a lot to this point. I spent years and several relationships wondering, agonising, convincing myself, and at times believing I was in love. Then I met someone and fell in love. All the questioning doubts no longer applied, all the romantic stories made sense. I didn't choose this feeling, or work it out. I 'just' felt it. It was there and it was something different from the loving relationships of my past. This was a new experience for me, a new emotion - 'real' love.
So to me there is something about love that is compelling (if we accept it) and therefore has the capacity to override more logical, sensible considerations like duration - 'it's only been 6 weeks, it can't be love', distance or some other incompatablity like financial status, nothing in common, etc. .
The trick is knowing yourself. I was blessed (cursed?) with an experience that left me with no doubt about the existence of deep love or whether I was feeling it (I was). And so that feeling, until I experience something else (which may be happening, watch this space...) is my benchmark for love. If it happens I know, without question,
But i also understand being unsure. Louis was only half right. And that NRE as its called can sure look and feel alot like love. If I am unsure, then I try to be honest with myself - simple but not easy - and see if I can filter out the bubbly giddiness, the crazed hornyness (if that is applicable) the 'i want to spend loads of my time with you, we fit together so well, this is amazing!' and if i am left with 'that benchmark feeling' then I am in love.