Depends how much in love she is. If its 100%..she teases, jokes and or hints at marriage, or weddings. She may even talk about kids. At the very least. you notice she is always talking about her long term future plans which includes you in it.
Also...she never puts up red flags...she never puts you down...especially in front of other people. She takes you to meet her parents, she asks to meet your parents...she invites you over, and cooks you dinner. She's never in a bad mood around you..always happy, and smiling. In her eyes...you cannot say or do anything wrong...ever. She never lies, or is deceitful. You never find yourself questioning her.
Remember..above is how she behaves if she's 100% in love with you...and ready for marriage. Now, you can use this to gauge how in love your girl is.