People do indeed take advantage of the kindness of others. The hard part is not letting that turn you into a cynic, which is more difficult than it sounds. Keeping your expectations low helps with that. Also, the more you interact with people like the homeless woman in your story, the more you’ll start to pick up on the signals that people like her tend to emit. With time, you’ll develop a much better sense of who to offer your help to, when to offer it, and how much you should offer.