Similar chemicals are often stored together in enclosed spaces with little thought of possible cross-contamination. In most cases, the contamination of trace levels of volatile organic compounds (VOCs) is not a significant cause for concern. Though humans often remain unaware of contamination, it is relevant in the case of canine scent training since a canine’s superior sense of smell makes them highly likely to detect contamination even at trace levels, which can contribute to inefficient training. Canine training aid kits contain multiple chemicals stored at a variety of temperatures and conditions that could lead to cross-contamination of co-inhabited training aids, and subsequent use of these aids may eventually result in less reliable canine units. Thus, herein we address the need for a simple, field-portable, vapochromic sensor to determine the cross-contamination of VOCs within canine training aid kits. We demonstrate the development of a vapor sensor, which produces a rapid colorimetric change when ammonium hydroxide is present. A pH indicator is used as the colorimetric dye and is incorporated into a sol-gel matrix on a paper substrate to increase vapochromic response and device sensitivity. Sensor preparation involved the optimization of the pH indicator solution and the integration of the indicator in the sol-gel matrix. The indicator’s colorimetric properties were translated to vapochromic capabilities by exposing the sensor to the vapors of the analyte of interest, instead of directly interacting with the aqueous analyte. The indicator reacts with a target analyte inside the canine training kit of interest to produce a rapid change in color, visible to the naked eye. This design will allow the sensor to present a clear and unambiguous visible response to the release of the volatile target within a closed container, which is indicative of possible cross-contamination of adjacent canine training aids. It can be readily incorporated into existing training