It is clear that accurate diagnosis can be carried out only by qualified specialist in a garage on professional equipment. But some failures in the car can be found on circumstantial evidence.
The condition of conventional spark plugs can tell about many things.
How should look like a spark plug, if the engine and fuel system work perfectly? Working head of such spark plug will be slightly blackened, the color of electrodes should not be different from the head, and there should be no deposits on the insulator.
The solid black head, electrodes and insulator talk about increased fuel consumption, and hence of incorrectly adjusted carburetor.
If the head of the spark plug is white without buildup and deposits, it means that in the fuel mixture the oxygen is insufficient and this causes the engine to overheat and break quickly.
If the electrode and insulator of spark plug have a reddish hue, many additives are in the fuel and spark may operate no longer.