The jumbo version of john has a
--list=format-details option. By default, it outputs one record per hash type for all hash types, with tab-separated values of a variety of information about that hash type (maximum password length, long name, etc.). You can also specify a hash type.
So for a specific hash type:
$ john --list=format-details --format=descrypt | cut -f15
The plaintext can vary - for hash types that have minimum lengths, etc. or if a particular failure mode is being tested for. In this case: