If a girl is buying running shoes, ask her if she runs. If she bought awesome bright neon orange sneakers (like I just did) ask her about them. Strangers ask me about my orange sneakers all the time.
Is she coming out of Spencer's or a similar store? Ask her if it's her favorite store. It's got some funny stuff.
What is she carrying? Do the items she is carrying lend themselves to hobbies? Or is she just a shopaholic?
I have a long time interest in psychology, specializing in relationship dynamics for 30 years.
(Please note, we give the best advice we can based on the information given in a post. For better advice, please include the age of all romantic partners.)