How can i tell if my friends using cocaine?

okay heres the deal my two BEST friends (they are brother &sister) have been using marijuana a lot for around a year and a half or so and I've ALWAYS hated this but i was finally getting use to the idea that i couldent stop them. All i could do was keep them away from other drugs and things seemed to be working up until i moved schools. 3 weeks after i moved to the new school the brother did cocaine and ensured me that his sister didnt do it. Even if she didnt do it im still afraid for him and his well being but at least he admitted to me hes doing that sh*t and what concerns me the most is if his sister starts using that to! id die if she starts using coke it would kill me. I havent seen them in a while since this happened im probably going to see them soon how can i tell if SHES tried coke will i be able to see it in her face or her body? and will i be able to tell if HE's done it more than once? i am really worried for these guys i care about them so so much can anybody PLEASE help figure this out???????