Okay, this was brilliant.
Linda Sue is explaining how just like chocolate chip cookies, the way you know you like one or not is because you've eaten a LOT of them. Like 'em crisp, or chewy, or soft, or with nuts? Because you've eaten so many cookies, you have an internal scale against which you can judge any cookie. Even if you've baked them, you know when you eat one if it's something you like, if it's good.
There is only one way to get similar judgement of your own writing: read A LOT.
Build that mental scale.
Read a thousand picture books, like Brenda Bowen suggested. Read 500 novels. But more than that, consider how long it takes to become an expert in other professions - doctors, lawyers, plumbers... all those people need to TRAIN. For years.
Our training is READING!
When Linda Sue writes a mediocre sentence, she knows because she has a vast storehouse of wonderful sentences that she's read in her mind.
We can each build that mental scale for ourselves. And maybe the next book we should all read is Linda Sue Park's latest novel, "A Long Walk To Water."