How do you catch a teen using marijuana?

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I was told that some of my students are using marijuana. However, I cannot do anything about it as I do not have evidence to support this claim. When I am around, the students usually behave normally. One would hardly think that they are using such stuff. But I do think that my informant is not lying because I have heard of others say some behavior-related problems regarding some students when my back is turned. How do I get evidence that they are using marijuana? I want to prevent them from turning into addicts. Thanks for your suggestions!