Listen to everything she says, and take notes.
A crucial aspect of buying great gifts is being a great listener. Chances are, she constantly mentions little things she saw that she liked, or wishes she could do just in passing. Within the last week, she has made offhand comments about losing her headphones, wishing she could see Future in concert, and liking a friend's red trench coat.
It's your job to catalog these inadvertent gift requests as they come up. Not mentally; make an actual note on your iPhone labeled "gifts" and write (er, type) these things as she mentions them.
If you haven't been doing this up to this point, it's not too late. Rack your brain for things she's mentioned in the past, and listen carefully in the upcoming weeks. You'll be amazed how many things she mentions that can act as clues to point you in the right direction.