Sort out/separate the sheep from the goats British American verb
Mom : How do you think about your classmates ? Son : I find it fairly hard to separate the sheep from the goats in the first encounter.
Examination is a way to sort out the sheep from the goats.
She is so naive that she can't separate the sheep from the goats.
The idiom derives from Bible. According to Biblical belief, on the Day of Judgement as well as the day the world ends, God will sort out the sheep from the goats. The sheep (symbolizes the good person) will go to heaven and the goat (embodies the bad one) will go to hell.
To write something
Yesterday, he put pen to paper and replied to all letters from his fans.