Damn you, AN, and your difficult questions.
I don't think there's some magical box that gets ticked when you're in love, so I can't provide an actual answer to your question, unfortunately. I think you can definitely see some
signs though:
>You're extremely comfortable with that person both with interaction but more importantly, in lack of action (i.e. can sit together in silence and not feel uncomfortable)
>You care about them for who they are not for what they can do for you
>You don't just wish to be with them when you're lonely, but when you're already happy
Things like these. I don't know, I'm not actually eloquent enough to describe it more than a feeling that
just is.
To answer the second question: I'm not in love right now, but I believe I was with my girlfriend/best friend for a bit under four years (not here to go into that though haha). The thing is, however, that while I
think I was in love back then, there's always that 0.1% doubt that perhaps I wasn't, and it's a different feeling altogether. Perhaps another question is whether you can ever be absolutely certain you're in love (although this is more of a 'in retrospect' thing rather than 'in the moment').
For those who aren't sure though, I think it's very important to think about the distinction between love and obsession (which aren't necessarily mutually exclusive)
Interested to read the other answers though -- especially ones that might answer the question more than I did
Edit: Try to imagine the feeling a dog gets when they're let out on a big open field