Being next to transport (except railway transport) and pressing the J key (Russian O), you can view all the extras of this transport (if it has them).
The second version added:
The extra number is not set by random selection from a given range, but all available options are sequentially sorted out;
The extra number is displayed on the screen;
When you try to view the extra number of a vehicle with a driver, a vehicle attached to another vehicle, or a railway vehicle, a message is displayed: does not work;
Added script source with explanations for students of GTA scripting;