Create Test plans which insure that ETL batch perform the required data transformations and load as specified im the requirement documentation .
most of the requirements are derived based on source to target mappings which is nothing but a document which specifies any business logic to perform while perfom ETL operation. example may be source has First Name and Last Name as two separate columns but Target databaae would need a single column as Name and that should be specified as First Name and Last name together.
