How do you know if someone is using meth?

I remember back when me and my bestie first met- we had soooo much fun and I enjoyed her company until the drugs others were doing around us influenced her to do the same. I know she had been doing adderall for a while then opiates often enough but she is being even more different than then. Shes never talked about meth and shes told me about every thing else. At least i think she has. Anyway is it possible she is using meth and how do you know if someone is?