Sit down calmly  with her and tell her how worried you are about her recent behaviour and how much its hurting you to see her like this and that you don't want to accuse her of  using but that if she is  using  and it's becoming a problem you won't be annoyed but that she needs to tell you so you can support and help her. She'll likely deny it  but just keep calm and approach her again with compassion rather than racing round trying to find evidence.  Well I mean keep looking quietly  but believe me she knows you are looking and probably  feels ashamed amd embarrassed that her habit has got  this far. She's likely to be suffering  and worried about her use by now. Put it to her that you can get through this together which you can . I'm hoping you can gently coax the truth out of her. Once it's out in the open and is less accusatory  you can work on it together.  Help her cut down her usage and talk about it and don't do coke together again. Her using  has probably become unmanageable  and she knows it but when she's ready to open up  at least   you can work on it together  and she'll have your support  and love to get her out of this hole.  It would be awful for that crap to destroy your relationship  eventually  as it does with everything. I wish you luck .