separate wheat from chaff
Sort the valuable from the worthless, as in I hope we'll get a preview of the auction so we can separate the wheat from the chaff. This idiom alludes to the ancient practice of winnowing grain.
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.