I wrote an article for a newsletter and submitted it to the editor. When I asked what she thought of it, she said, "I thought this was written for me!" I chuckled and said that I have that effect on people. But what was so amusing was that the article was actually written for me.
The article was about honesty and how, no matter how difficult and scary it can be, being completely upfront and honest will always lead toward true happiness. It was something that I have come to realize from examining my own life and the lives of people around me. It was just as much a self-affirmation as it was advice to the masses.
But I think that's how I am sure that it's a good piece of writing. Good writing is universal (or as close to it as one can get). It speaks to common life experiences, but does not do it in a vague or hollow way. When somebody reads your writing and thinks that it was written for them, you know you've got something. And when you think about it, it was written for them.