You'll need to explain in more detail.
1) When would you want the system to check for an empty field? When you exit a newly created row in the portal?
2) What input fields? What calculated output field? How might it be "wrong"?
You can certainly set up a custom validation message that will appear if a field is left empty if you set up the field to be required in the field validation settings. You can also set up a script trigger such as OnObjectValidate to display a custom dialog if the field is empty.