A simple and novel technique, the insecticide-residue indicator drop (i-RID) method, was developed to detect insecticide residues on-farm, at market, or before cooking, based on the surface tension of a coloured water drop. Eight insecticides such as Carbofuran, Chlorpyriphos, Dimethoate, Fipronil, Profenofos, Spiromesifen, Thiamethoxam and Triazophos were sprayed on brinjal in the field, and fruits were sampled at 1–10 days after treatment (DAT). A drop of water, coloured with Congo red dye for improved visibility was placed on the blossom end of each collected fruit, and the spread area (mm²) was measured through a transparent grid placed between glass lids. Insecticide residues lowered the surface tension of the drop, causing it to spread more than on residue-free control fruit, where drops remained small and globular (9.37 ± 0.03 mm²) throughout the 10-day observation period. On the first day after treatment, the maximum spread was recorded on triazophos-treated fruits (18.03 ± 0.43 mm²), followed by fipronil (17.59 ± 0.65 mm²) and thiamethoxam (17.21 ± 0.37 mm²), while the least spread among treated fruits was observed with carbofuran (12.48 ± 0.40 mm²). Residue detectability persisted up to 6 DAT in profenofos-treated fruits, whereas the remaining insecticides showed detectable residues only up to 3–5 DAT. These findings indicate that the i-RID method can effectively differentiate insecticide-treated fruits from untreated ones and estimate residue persistence over time. Being simple, rapid, and inexpensive, this technique holds promise as a practical on-site tool for farmers, traders, and consumers to screen insecticide residues on brinjal before harvest, sale, or consumption.