Web Services response return only those fields that are not empty.
Therefore it's not possible to use WebServices response to synchronize newly added fields with an external application.
Note that an unchecked checkbox is being returned with a false value. On the other hand, other fields types such as free-form text, will not be part of response if it is left blank on the record. For example, if the Phone field on a customer record is blank, the said field will not be part of the SOAP response