I have asked her in the past if she has ever thought about it and she said no, but if she does want me to do it, how do i know she does or how would I even do it at all?
There are several factors to consider -
1) do you want to give her oral sex
2) has she ever had oral sex
If you like the idea of pleasing her, and she has not indicated any negative feelings about it, at the next opportunity just slide down and give her some. At a young age it's likely her opinions about such things may come from second hand stories and not personal experience.