I'm not sure I understand your question. Form 1040X does not tell you what the underlying main form is. If your parents used TurboTax, the PDF that contains their Form 1040X amended return will also contain the revised main form. You have to find the main form in the PDF to see whether it is 1040 or 1040A. If they did not use TurboTax, how you find out what the main form is will depend on how their amended return was prepared.
