Mass spectrometry is frequently introduced to undergraduate students as an instrument for both qualitative and quantitative chemical analysis. One of the most common uses of mass spectrometry (MS) is to deduce or confirm a compound's chemical formula, by relying on high-resolution accurate-mass measurements. However, like all forms of instrumental analysis, qualitative characterization of an unknown entails a degree of measurement uncertainty. By extension, the conclusions of any analysis may not be definitive. Here, we describe a form of inquiry-based learning wherein students acquire scientific evidence to probe a question ("Is the person guilty?"). A laboratory-based experiment involves the use of accurate-mass analysis to characterize a trace unknown, extracted from "simulated" forensic evidence. Together with the accurate mass, the isotopic distribution, some chemical intuition, and an online molecular-formula tool (available through ChemCalc), students arrive at the suspected chemical identity of the unknown. The analysis of cocaine on circulating currency is used in this report to illustrate the lab exercise. At the conclusion of the exercise, students should realize that the evidence they generate is insufficient to support a definitive identification. The strategies employed to deduce a molecular formula may also be adopted to a classroom setting.