@Gotterkraft
Ideally, when you set the form objects as Required i.e "Type: User entered required" in a PDF Form then ideally, the user won't be able to submit the form without filling the required fields.
In your case, if this is a requirement then you can put your validation code[0] on preSave event to warn the user that the form is not complete. Please be informed that this may just notify them if there are any missing fields, not stop them from saving.
Just another tip, all required fields are marked as <validate nullTest="error"/> in the XML source.
[0]-
Page1.execValidate();