Hmmm. When I make myself emotionally vulnerable for you. This is not the easiest of tasks for me, so if I do this, I most likely care for you. I think I've only been in this situation once, though.
"A spill at the plant increased the phosphates in the lake and produced a scum of algae so thick that the swamp smell filled the air, infiltrating the genteel mansions. Debutantes cried over the misfortune of coming out in a season everyone would remember for its bad smell."
If you're getting a lot of my time. I tend to be very choosy with what I do with my time. If I'm giving up a good chunk of my precious "battery recharge/introvert" time and Ti time to be with you, then I'm digging you. I would say, generally, this doesn't apply to the first 6 months of getting to know someone. Generally if I meet someone interesting, I like to hang out and get to know them for a period of time. But, when that initial 6 month "warm-up" period is over, and I'm still thinking about you and wanting to be with you, then it's a good sign that I might kind of have the hots for you.
"How dreadful!" cried Lord Henry. "I can stand brute force, but brute reason is quite unbearable. There is something unfair about its use. It is hitting below the intellect." ~ Oscar Wilde - The picture of Dorian Gray