You have to be aware of the market you're writing for, and think about it objectively. Here's a funny little example, from last week, in my own writing.
I am working on a middle reader novel and the Story Monkeys are examining it as I write. One of the Story Monkeys suggested that he thought my sentences were too complex for the readership.
Q: how to test?
A: I picked up off my shelf two of the best-selling middle reader books in the last decade (I will give you a hint -- the protagonists' names rhyme with Shmarry Cotter and Shmercy Slackson), took a significant chunk of pages in each and counted commas. Over a similarly sized sample, I kept track of how many two-comma sentences each had, and how many two+-comma sentences. Turns out the two are quite close in comma frequency.
The Story Monkey in question was absolutely right. Looking at the big sellers in the market not only confirmed he was right, it quantified it, and helped me figure out how to calibrate my own sentences.
Moral: get a good writing group.
Moral: examine books in the market for which you are writing.
Moral: Story Monkeys, ho!