An empirical test to detect potato tubers susceptible to after-cooking darkening was developed from a histological method for revealing chlorogenic acid (CA). Since CA is the predominant compound responsible for after-cooking darkening, the test provides a simple field technique for use by potato breeders and food processors. No cooking is required, but suitable precautions must be taken with the reagents. The test kit uses two dry reagents: (1) a mixture of two parts urea with one part tartaric acid (wt/wt); and (2) sodium nitrite. Three 0.5 g scoops of the urea-tartaric acid mix are dissolved in 60 ml of tap water. Two scoops of sodium nitrite are added, then the solution is poured over potato slices. After 5 min, the slices are transferred to 2% sodium hydroxide for 2 min to develop a red colour where CA is present. Potatoes with unacceptable after-cooking darkening show CA deeper than 3–4 mm from the surface, either as surface staining 10 mm deep, or as pink coloration throughout the tuber. High CA levels just under the skin would be removed during peeling, so the total amount of CA is not as important as its localisation.