If you're married, or have a significant other, you have probable been on some form of Saturday morning outing that has nothing to do with cars. Often including garage sales and antiques, you have probably been to a "country" store. One popular item is the "piece of string weather station". You know, "if the string is wet , it's raining if it's dry it's sunny" etc.
Reading spark plugs, especially on pump gas, is kinda like that. If your plugs have any visible residue on them( that is NOT metal), you "might" be rich. If they look clean, you might be "close". If the electrode is missing, you are "probably" lean. If they are wet, it's raining....Oh man is it raining!
Reading plugs takes a long time to learn, long time to do and probably shouldn't be attempted with the naked eye.Have someone with experience teach you to understand what you are seeing and NEVER tune with this technique unless you know what an "oh,no! she's about to blow!" plug looks like before hand.