An infusion set or patch pump is provided for delivering a pharmaceutical agent, such as insulin, to a patient. The infusion set or patch pump has a recessed portion in the bottom face containing a hydrogel and a reactant capable of producing a color change upon contact with the pharmaceutical agent. The reactant includes a mixture of potassium persulfate and 4-aminoantipyrine and optionally horseradish peroxidase, and produces a rapid visually detectable color change when contacted with phenol and/or m-cresol contained in insulin as stabilizing agents.