Tigergirl has a very good list.

I know my own predilections, haha! For me, here it is, when someone doesn't care I get a very nervous panicked feeling, even in casual situations. I think you can tell when someone cares because it's the opposite. It's like disconnected, you could be anyone to them, they have a bored/ glazed look. There is a radar inside us that picks it up. If someone gives you that general comfortable feeling inside, then there's a pretty good indication right there that they care. Basically if you feel good telling them about your issues.....