By paying attention to patterns. That's the only way to determine if a person cares about you or not.

By patterns, I mean; how do they respond to you when different kinds of situations arise? Do they support you? Or, do they invalidate you? Are your friendships and romantic relationships transactional in that you and the other person mutually support each other? Or, are your interpersonal relationships all one-sided; you do something for someone else, hoping that will make them like you and want to help you out in return?

If you have any one-sided interpersonal relationships, get rid of those as they are not good for you. If you have transactional interpersonal relationships -- where you and the other person involved mutually support and respect each other, those are the relationships that you wang to hang on to and nurture.