it's bad reporting and media narratives that people cut meth and coke with fentanyl. It does happen, but is far from some common thing. it's mostly accidentally, or contamination. In some cases, people thought one drug was the other, like bought an ounce of "coke", that was really fentanyl, and didn't know the difference. You just have to read more in depth about it, and not headlines,
when they do autopsies, they can't really tell the difference, but sometimes they can piece it together from the paraphranila the user leaves behind