Field Form Rule Script - Only execute if all mandatory fields are entered

Is there a way to check if all mandatory fields are completed without checking each field individually?

I have a Field Form Rule script that executes on create and adds text in the description field. Currently, if the user missed a mandatory field and clicked Submit, the field form rule runs, then notifies the user that they missed a field. Then when they update the field and click submit again, the additional text is added a second time. I only want this text to be added once.

Is there a way to only run the script when all mandatory fields are entered? I don't want to validate every single field individually because there are several fields and the mandatory status is conditional based on other field values.