How to judge whether the working state of the spark plug is ''healthy''?

If you want to check whether the spark plugs are healthy, the most direct way is to take them out and have a look. If the electrodes and insulators are light yellow, it means that the spark plugs are in good working condition. If there are black deposits on the electrodes of the spark plugs, it is "sick". It may be caused by "burning engine oil" or abnormal gas mixture ratio.