Ask her! You can't improve anything, including sex without information, learn your partner, talk to her!
How can I knew that my girl is happy when we am having intercourse, and if she is enjoying the sex?
Know her body language. Does she look like she's enjoying it and trying as hard as she can not to moan from pleasure? Or does she sound like one of those banshees from a p0rn0 movie?
Talk with her. Let her know her pleasure during times of intimacy is really important to you. Oh. And make sure she has a v1brator. Have her use it before and during. Maybe even after.