Love is at that intersection of friendship and passion.
A friend is someone who makes you happy and makes you laugh, someone you can trust, someone who understand all of your states - silly, funny, sad, upset, someone who will be there for you whenever you need, someone who cares about what you say and think, someone who shares your victories and losses.
Passion is when you desire them, when your body calls for theirs to be on yours, when you want to be taken by them and only them. You want to make love. You want to fuck.
Love is both coming together, to be expressed with emotions, upheld by your shared beliefs and morals, made everlasting through daily commitment and thoughtfulness, through little and big gestures.
That's how you know - when all those things are visible, present.
Sitting in my kitchen nook by myself.
Will you eat cake tomorrow?