To verify that a detonation occurs in a pulse detonation engine, pressure sensors have been used to measure the flame propagation velocity. However, pressure sensors are easily destroyed and quite expensive. An alternative method has therefore been examined in this project where the propagation of the flame is measured by detecting the time when the conductivity between the spark plug electrodes changes. The velocity is decided by the time intervals between the signals from several spark plugs, at known distances, placed after one another. The results are compared with the results from measurements using pressure sensors. The use of spark plugs instead of pressure sensors to verify detonations showed to be an equally good as well as cheaper alternative. The difference from pressure sensors is that spark plugs record the combustion wave instead of the pressure wave.