How do I find out if my sister is abusing drugs?

My sister is 16 years old and has recently been going out to hangout with friends every weekend, which is fine, but I found out that one weekend she wasn’t where she was supposed to be. She told me she was with a boy and that he was about 26 years old, and that they had slept together. A few weeks have went by... show more My sister is 16 years old and has recently been going out to hangout with friends every weekend, which is fine, but I found out that one weekend she wasn’t where she was supposed to be. She told me she was with a boy and that he was about 26 years old, and that they had slept together.

A few weeks have went by and I now know that he’s actually 34, as if him being 26 wasn’t already bad enough, and I found out that the weekend that they hung out my sister’s friend’s family had kicked him out of the house after realizing that he had meth on him. Rumors have circled around school that my sister had told a friend that she had tried it but I don’t have any solid proof and I want to believe her when she says that she hasn’t but she’s lost my trust. (She was drug tested by my older brother but it had been weeks later so it came back negative) On top of that, this guy has several drug and domestic violence charges against him.
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