Kyle, Just a stab.
Count ( related::parentID ) should tell you how many portal rows. Count ( related::myTestField ) should give you the non-empty count for that field. If the two numbers don't match then you have some empty fields.
Note I'm counting the parent match key in the related first as it would never be empty in any portal row, even if all other fields are blank. If it's 0 no need to count the other - you don't have a match.
If your portal is filtered, then there are other ways (smile).