Self-esteem vs Ego

I've noticed many people stumble over the idea of self-esteem vs ego.  Its a key point in "A New Earth".  I enjoy reading Carolyn Hax' column and I like an answer she gave to a question about the difference between ego and self-esteem. 

QUESTION:  What is the difference between ego and self-esteem?  "Ego" seems to generate all sorts of problems, whereas everyone needs good "self-esteem".

ANSWER:  "Ego" can have several meanings, depending on context.  If you're discussing "ego" as a part of someone's psychic anatomy that just got bruised, then it's interchangeable with "self-esteem."  "Self" is another definition of ego, and Freud had his own definition.
I suspect the guidance you're looking for isn't semantic, though.  It's emotional:  When does good self-esteem go bad?
Ego and self-esteem are the bond between who you are and who you perceive yourself to be.  When your ego (or self-esteem) is healthy, you have a realistic idea of your strengths and weaknesses.  When your ego (or self-esteem) is unhealthy, there's a breakdown in one of two directions - you have significantly more, or less, going for you than you think.
To asses your own health, think of ego as the security guard while self-esteem is the institution it protects.  If the institution - the self - stands soundly on its own, built by hard work, a sense of purpose and a sense of accomplishment, then it doesn't need to hide behind a huge security/ego system to protect it. The result is a person who can shrug off failure, say, or rejections, or who can perform a so-called menial chore without fear of being lessened by it.  Its not needing a constant supply of approbation.  It's what people mean when they say, "S/he has no ego."
When the institution of self is shaky - when a person is praise-dependent, or feels unworthy - that's when it's paramount to appear strong.  Enter the ego staff, either to gin up bravado or to churn out preemptive apologies; to create and shore up your appearance of invulnerability, or to tear you down before anyone else gets the chance to; to decline to admit being wrong, or to scour for new ways you can blame yourself; to attempt to controls others, or to latch on to someone else; to chase away people who might get to know you too well.
Gauging your health may be as simple as checking your own maintenance program for lack of a better term.