A method to estimate the propagation speed of a three-dimensional ignition front in Direct Numerical Simulation (DNS) is discussed. The objective is to contribute to the design of advanced numerical tools for the study of sporadic pre-ignition kernels leading to violent pressure waves, which may for instance damage moving parts in engines. Estimating the speed of a propagating ignition front in three-dimensional DNS, before it is occurring, is not an easy task, because this speed scales as the inverse of the spatial gradient of the time left till ignition, which is a priori an unknown quantity. The proposed approach introduces, for every point of the DNS, an estimation of the time left till ignition, which is obtained from reactors dynamically parameterized from the time evolving DNS results. This provides a three-dimensional distribution of ignition delays at every instant in time of the DNS fields. The time evolution of the ignition speed is then computed from the space derivative of the ignition delay field. Only the pre-ignition phase is examined and the demonstration of the method is made with oversimplified chemistry, in order to apply it to an existing rapid compression machine, in which the mixture composition is homogeneous, whereas the temperature distribution is non-uniform due to heat-transfer at wall. The two expected distinct ignition regimes are reported. In the first, ignition propagates at the speed of sound, or even above, and occurs over a large portion of the combustion chamber, with a strong and sudden pressure increase. The second ignition regime is much more localized in space and with a propagation mechanism pertaining to a deflagration mode. A method is also discussed to delineate in the DNS between ignition influenced by either the constant pressure or the constant volume canonical behaviors.