As the social service providers handed out fruit snacks and water bottles on Thursday, down the block from a needle exchange on Berkeley’s University Avenue, they asked passers-by an unusual question: Did they want to “check” their drugs?

Naila Vitatoe explained the operation to a middle-aged man. She works for an Oakland nonprofit that analyzes street drugs for unwanted chemicals — a dash of fentanyl in a bag of tar heroin, for instance, or an emerging industrial chemical that sears the veins of users.

Vitatoe and her colleagues take a small sample and test it with a machine that beams infrared light. Within minutes, they return the drug to the user, along with information about its true contents and safety advice — all free of charge.

The man left and reappeared five minutes later wit

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