There's no sure way to know, and it may not be possible to tell for sure. However, if your partner seems more interested in gratifying themselves than in giving you pleasure, it's likely that they're humping you for satisfaction rather than desire. If they're constantly trying to get you off without focusing on their own needs, or if they never seem to reach orgasm themselves during sex, those are both signs that they might be using you for their own satisfaction.
It's important to communicate with your partner about what you both want and need from sex. If you're not enjoying yourself or feeling fulfilled, talk to your partner about it. They may not realize that they're not satisfying you and may be open to making changes. Ultimately, though, only you can decide whether or not your sexual relationship is meeting your needs.