That is a really good question. I guess it really depends. I think, when you can argue with a person and still care for them; when you don't want to hold onto the anger and you just want to hold them, you are in love. When you both like your time apart but look forward to time together and know that you can lean on each other in harder times, I think that's a good indication of love too. When cuddling doesn't always lead to sex and you're ok with that, or you can trust them with your heart and you respect each other...man this is mushy....I think you just know.

I guess if you can't really imagine yourself spending your life with anyone else. But give it two-three months because it could still be a crush, but crushes don't last that long.

The way love works on your brain, it'd release some chemicals due to which, when that person is around, everything will seem much better, and you will want more of them being around.

And, of course, other things. You'll have delusions of your strength and bravery around them (making them happy, or something related, generally, to be more specific), and you'll feel less of your pain. They'd be causing you to release big doses of dopamine on your brain, the reward hormone, which makes you feel better about yourself, and norepinephrine, which is adrenaline, the stress hormone. It'd make you much more responsive and aware.

(The state is almost entirely similar to being high on cocaine.)

Additional resources:-

http://www.iep.utm.edu/love/

http://neuro.hms.harvard.edu/harvard-mahoney-neuroscience-institute/brain-newsletter/and-brain-series/love-and-brain

https://www.psychologytoday.com/blog/neuronarrative/201402/what-neuroscience-tells-us-about-being-in-love