Filter
Feelings are complicated, but how does the person make you feel? That's kind of where I'd start. Do I think about them randomly? Maybe see something or do something that I want to share with them? When I think about them, what are my thoughts and feelings? Good, happy, giddy, light, silly? Do I want to spend time with them and get to know them? And then I start thinking about what if we were more than friends? Can I see myself going on a date with this person? Do I want to hold this person's hand? Kiss them? Hug them? Make them laugh and smile? How would I feel if they dated someone else or liked someone else in the way I like them? Why do I like them? What about them has them in my thoughts and feelings? Have you ever liked someone before? How does it compare?
It's a lot of self reflection, and it's messy, and it's complicated. There's a lot of physiological changes that can happen when you like someone, but I don't think you want that. I can tell you about them if you do, just in case, but I think liking someone can be very personal to each person and then catered further by the person being liked.
But it's also hard trying to find that line of "I like you as a friend/best friend" and "I like you as more than a friend/best friend." I think, for me, it comes down to how they make me feel. And maybe turning over the idea of being more than friends to see what it might feel like.