Once upon a time, “well” and “good” were not to be used interchangeably.
Particularly when answering the question above, “good” could be used to indicate virtue. Think of it like a short way of saying “good deeds” or “good things.”
“Well” more often referred to your well-being. If you weren’t doing well, perhaps you were doing poorly, or just OK.
The rules are looser these days, but one often sounds better than the other.
My advice is to take it back to primary school: When in doubt, sound it out. In other words, try each word in the sentence, and see which sounds better.
A few examples:
- “The bartender made the martinis so well.” In this case, “well” is an adverb describing the action — the making, or the degree to which the bartender was successful in making the drinks. You could argue that “good” sounds OK here, but I prefer the traditional word choice.
- “These are good martinis!” Here, we are describing the taste of the drinks, not anyone’s ability to make them. We’re using “good” as an adjective. “Well” would not work in this case.
- “Things are going good.” I’m 50/50 on this one. Both “good” and “well” work here. ChanteSez this is a matter of formality, the difference between talking among friends (“good”), and talking to your boss (“well”).
ChanteSez.com 
I think this one is even simpler:
“Well” is an adverb. You DO or ARE something “well”. Other examples of adverbs are “quickly” and “badly” and just about any other “–ly” word that you would apply to a VERB.
“Good” is an adjective. A thing (noun) can have the QUALITY of being “good”. Other examples of adjectives are “red” and “heavy” and just about any other word that you would apply to a NOUN.
I don’t think “sounding it out” works — people that are saying it wrong will think it sounds fine. How you doin’? I’m doin’ good! 🙂