try asking. she might know. If you have trouble asking why are you even in this relationship? Also try starting very slow and starting far away from the genitals at the extremities gradually getting there. Just general arousal info.
The g-spot is on the upside of the vagina, the side facing you. You can't touch it in the missionary position. You have her legs wrapped around you or above her with her knees nearer to her stomache.