how do you check if /etc/shadow is corrupt?

NAME
       pwck - verify integrity of password files

SYNOPSIS
       pwck [-q] [-s] [passwd shadow]

       pwck [-q] [-r] [passwd shadow]

DESCRIPTION
       pwck verifies the integrity of the system authentication
       information. All entries in the /etc/passwd and /etc/shadow are
       checked to see that the entry has the proper format and valid data
       in each field. The user is prompted to delete entries that are
       improperly formatted or which have other uncorrectable errors.

       Checks are made to verify that each entry has:

       ·  the correct number of fields

       ·  a unique user name

       ·  a valid user and group identifier

       ·  a valid primary group

       ·  a valid home directory

       ·  a valid login shell

The checks for correct number of fields and unique user name are fatal. If
the entry has the wrong number of fields, the user will be prompted to
delete the entire line. If the user does not answer affirmatively, all
further checks are bypassed. An entry with a duplicated user name is
prompted for deletion, but the remaining checks will still be made. All
other errors are warning and the user is encouraged to run the usermod
command to correct the error.

The commands which operate on the /etc/passwd file are not able to alter
corrupted or duplicated entries.  pwck should be used in those
circumstances to remove the offending entry.