Use a timing light & see if it flashes. Current through the wire is what flashes the light. Hold the boot with something insulated with terminal near the plug & will jump across the gap while...
To answer this question it would be helpful to have more information for example if your vehicle is older with high miles a worn out distributer can cause this problem
Faulty coil, or coil pack (depending on the vehicle). If your vehicle is quite old it might have a mechanical distributor (looks like a miniature milking machine with plug wires coming out of the...