This is the core problem, I rarely ever hear of someone using fentanyl by itself. Fentanyl is incredibly potent which maximizes profit/volume for traffickers. This creates a tremendous financial incentive for them to adopt since it provides a competitive advantage. If it's easier and cheaper to obtain fentanyl than Alprazolam (xanax), then illicit manufacturers will produce a counterfeit using fentanyl as the active ingredient. Fentanyl overdoses are also frequently caused by improper mixing techniques (powder to powder) and the non-uniform distribution will result in someone getting a hot dose. In that case, testing might not even help.