Modern electronically controlled passenger vehicle engines require use of a scan tool to display ignition timing.
A reference pointer is attached to the flywheel housing or other fixed point, and an engraved scale gives the offset between the spark time and the top dead centre position of the piston in the cylinder.
The distributor can be rotated slightly until the reference pointer aligns with the specified point on the timing scale.
Simple timing lights may just contain a neon lamp operated by the energy provided by the ignition circuit.
Self-contained units used to time automotive engines have an inductive pickup that clamps around the proper spark plug wire and serves as the trigger for the strobe.