Authentic samples of commercially produced organic and conventionally grown tomatoes, lettuces, and carrots were collected and analyzed for their δ15N composition in order to assemble datasets to establish if there are any systematic differences in nitrogen isotope composition due to the method of production. The tomato and lettuce datasets suggest that the different types of fertilizer commonly used in organic and conventional systems result in differences in the nitrogen isotope composition of these crops. A mean δ15N value of 8.1‰ was found for the organically grown tomatoes compared with a mean value of −0.1‰ for those grown conventionally. The organically grown lettuces had a mean value of 7.6‰ compared with a mean value of 2.9‰ for the conventionally grown lettuces. The mean value for organic carrots was not significantly different from the mean value for those grown conventionally. Overlap between the δ15N values of the organic and conventional datasets (for both tomatoes and lettuces) means that it is necessary to employ a statistical methodology to try and classify a randomly analyzed “off the shelf” sample as organic/conventional, and such an approach is demonstrated. Overall, the study suggests that nitrogen isotope analysis could be used to provide useful “intelligence” to help detect the substitution of certain organic crop types with their conventional counterparts. However, δ15N analysis of a “test sample” will not provide unequivocal evidence as to whether synthetic fertilizers have been used on the crop but could, for example, in a situation when there is suspicion that mislabeling of conventionally grown crops as “organic” is occurring, be used to provide supporting evidence.