How do women tell whether or not you are doing what you want because you want to do it, as opposed to doing things because of other people?

Something I've been thinking about is, I know that it's important for a female to know that a man is doing things 100% because he wants to do them, not because of some outside influence, and that he is free. Which makes me wonder, how exactly does a woman determine that you are doing something for yourself? and are free from others influence? How is this communicated?