Using a current clamp around the battery cable or starter positive wire. A good PDF will show you the mathematical integration of current draw. Each compression stroke creates a current peak. A low peak = low mechanical compression. This is a 30-second test that scan tools cannot perform.
Look for a sharp vertical drop to ground (0V) when the ECU turns the injector on, followed by a high inductive "kick" or spike when the circuit is opened. pdf automotive oscilloscopes waveform analysis