How to differentiate between someone loving you, and someone needing you? Or are they one in the same?

If you're in a relationship with someone, and it looks like love, how can you know the sincerity? How can you tell the difference between that person genuinely loving you, and that person needing you? Or is loving someone something that goes hand-in-hand with needing them? Even if the second party believes that they love you, how can you be sure that it's not the fact that you're the constant in their life that's making them think so? I often get confused as to wether my partner is right for me because I question wether his love is based purely on my nurturing him.