Are you doing this in the script? Try using "where not exists(field1, field2)" after a load.
If you are doing it in the document itself, eg a in a table chart, then I would use a test on a value in a field in table b that you always know will be populated - for example an id field (although you may need to create it).
An example would be =if(isnull(B_ID),.....). You could also do this in script if you wanted.
Is this useful?