Rather than spying, don't you think it would be better to discuss your fears with your daughter.
If she finds out (and it's very likely she will) about your subterfuge, her respect for you will plummet to depths so low that you'll have difficulty retrieving yourself.
Obviously I can't tell you how you should behave towards your children but speaking from my own experiences, I can discuss anything, anything at all, with either or both of my parents. There would never have been any secretiveness from either side when I lived at home and there isn't now.
Just my view.