I'd say you can just feel it. You start a relationship, follow it where it takes you and at some point, I feel that you just know in the back of your head. If you have to question yourself if you like somebody while you're in a current relationship, then I would consider that the sign that the relationship is not based off of love but something else, whatever that may be.
I agree that you can just feel it. But not about liking others, though, I am polyamorous and am in a triangle poly relationship, and I love both partners, it was really tough to start with, because I became friends with one, and I instantly took a shine to them, first time we went out just to hang out went pretty far, but to cut a long story short we're all happy and we care for each other.
I think love is tough to explain, and that it itself is tough. the three of us have depression and it's really hard sometimes, and one has psychosis and it's really tough for him, and I mean it's REALLY tough for him, and I really wish I could help him, I really want him to be alright. Anyway, what I'm trying to say is we have our flaws and we don't pretend they're not there, we love each other for who we are.