Drug checking is an evidence-based strategy for overdose prevention that continues to operate (where it operates) in a legal “gray zone” due to the legal classification of some drug checking tools as drug paraphernalia—the purview of law enforcement, not public health. This article takes the emergence of fentanyl in the U.S. drug supply as a starting point for examining two closely related questions about drug checking and drug market expertise. First, how is the epistemic authority of law enforcement over the material realities of the drug market produced? Second, in the context of that authority, what are the socio-political implications of technologically advanced drug checking instruments in the hands of people who use drugs? The expertise that people who use drugs maintain about the nature of illicit drug market and how to navigate the illicit drug supply has long been discounted as untrustworthy, irrational, or otherwise invalid. Yet, increased access to drug checking tools has the potential to afford the knowledge produced by people who use drugs a technological validity it has never before enjoyed. In this article, I engage with theories of knowledge production and ontological standpoint from the field of science, technology, and society studies to examine how law enforcement produces and maintains epistemic authority over the illicit drug market and to explore how drug checking technologies enable new forms of knowledge production. I argue that drug checking be viewed as a form of social resistance against law enforcement’s epistemological authority and as a refuge against the harms produced by drug criminalization.