The technical terms are thresh or winnow but I have never heard these used metaphorically. The metaphorical term is separate the wheat from the chaff.
Edit Having looked at other people's suggestions, I see that some suggestions, such as prune, involve taking out the bad bits. Others, such as distil, involve taking out the good bits. Separating the wheat from the chaff involves going through everything and sorting into two piles. It also seems to me that it is more specifically sorting the wanted from the unwanted, rather than making a judgment about good and bad. A typical example would be going through job applications where you go through them sorting into a pile of rejects and a pile that you will invite for interview.