In my opinion, to detect someone's secret, I think this kind of action is very very unprincipled, especially your friend!
Even if you detect your friend's secret due to your curiosity, also your action is incorrect, as the basic manners is defer to each other between you and your friend. If you don't do that, then you may hurt your friend, even lose your friend accordingly! If you do that only for a secret, Do you think it is worthy?
As a friend, if your friend has some secrets what is you don't know, you never ask her. also if you had been know her secret, you many feign you don't know, Don't mention about it any more! If you are a good friend, if she want to tell you, she always will tell you some day, if not, certainly, she has her reason! Don't detect it ! Defer to your friend, at first, you must defer to her intimacy!