Is my friend on drugs?

Hello everyone,
This is not exactly a health question and I wasn't sure where I should post it, but it is weighting on my mind and driving me crazy... I have a friend I am really close to and I suspect she is "snorting coke". I have asked her a couple times and she swears to me she is not. She knows that i am totally against drugs and she probably thinks that I would be upset with her if she told me yes, which I would but I only want to help her. I don't suspect shes doing it all the time but its like she goes threw phases every couple months. I see this friend almost every day. I have suspected it three different times because for about two weeks her attitude will change, she will have nose bleeds out of no where, she has the sniffles a lot and messes with her nose a lot, and she hangs out with people that I know do it. Like I said it seems like she goes threw phases, like the first time I suspected it went on for about two weeks and then things were fine for about two months and it repeated like this twice. Can anyone help me out with this? Are these real signs that she is using drugs? How can i talk to her and get her to tell me the truth is she really is? And how can I help her get off this drug if she is in deed on it?
Any advice and opinions will help! Thanks!