I think my friend does drugs, how can i find out?

i have this friend and like shes very very secretive. ive been her friend since the beginning of 7th grade and now were both in 9nt grade. she always been very like to herself and doesnt talk about her life at all. and if i ask her how her boyfriend is she says"omg could you be any freaking louder? dont say that out luod!!" and i think thats really weird becasue she wont tell me about her boyfriend. and lately shes told me that shes really dizzy. and she always has dark circles under her eyes. and her face has changed alot. and when i tryed to look at her new cute bag she freaked out and said dont look in there and every time i ask her if she does weed,meth,crack etc she says no then cracks up hysterically theni said you promise me,and shes like no i cant promise you. so she kinda made it obvious that she does but im not sure. it really scares me.