The crux of this is it IS her job to tell you what her need is. All people are responsible for expressing their own wants and needs to others. The only way you're going to understand is through her communicating it and developing emotionally. Granted, she's probably told you/verbalized it before, that doesn't mean it really "clicked" for you or truly understood. Other comments on the 5 Love languages are also really great suggestions as everyone receives and shows love differently.