Natural Gas used in high-efficiency engines holds promise as a low-cost intermediate solution to reduce Greenhouse Gases and particulate matter. However, to achieve high engine efficiencies, engines need to be operated at increased Brake Mean Effective Pressures (BMEP), which is limited by destructive, engine damaging knock. Alternatively, if controlled, the same End-Gas Autoignition (EGAI) process responsible for knock can boost efficiencies and consume unburned methane while leveraging low-cost traditional exhaust aftertreatment technologies, such as a three-way catalyst, to minimize environmental impact. For this reason, this work has developed a method to detect the presence of EGAI and to determine its onset location (or crank angle). This method is based on the characteristic profile of the Apparent Heat Release Rate (AHRR) curve under four levels of EGAI, where, by searching for the presence and location of inflection points, one can determine the onset location and intensity of EGAI. Results show a remarkably good agreement in determining the onset of EGAI for various engine operating conditions, including engine speed, IMEP, Exhaust Gas Recirculation (EGR) levels, and engine sizes, especially in the light/low EGAI levels that are highly important for a Controlled EGAI (C-EGAI) operation. The method can also successfully determine if a cycle is autoigniting or not and run under two milliseconds on a standard workstation, which could allow it to be implemented on a cycle-by-cycle real-time engine control setup.