You could get a reading eggs trial subscription and let her read and do the comprehension quizzes for different lexile levels. My twins used it briefly this summer, and it was immediately apparent when they chose books at too high of a level, especially when they took the quizzes.

Also, you can make a list of the books she likes and level them by using scholastic or a search engined. I did this yesterday for one of my sons, and found the books he chose to read were rated fairly consistently.