C.C.
As a high school teacher, I would hope that the parents of my students would snoop on their children. If you pay her cell phone bill, you have every right to go through her text messages. If she doesn't like it she can get her own plan when she is old enough to sign the contract without any help.
However, if she doesn't tell you much at all, maybe it would help to reconnect with her & have a mother/daughter date on a regular basis - go shopping, go out for coffee...do something that lets her know you want to be part of her life & it helps to foster a friendship within the parenting as well. It might put her more at ease & she will "trust" you more with those "secrets" she is too embarrassed to share.
Good luck!