 They come to the van, they know who we are, and they just grab their supplies and they know. To test a stimulant for fentanyl, you take a tiny crumb and mix it with a little water. Then you dip in the test strip for a full minute. I wanted to see for myself, so I went to a neighborhood where a lot of drug users hang out. I met a chatty middle-aged Latino man, J.R. NPR is using his initials because he uses illegal drugs.