Normally, the SEF is able to understand this situation ex officio, by verifying the payment of taxes and Social Security contributions (when applicable) over the years. Even so, if they are unable to determine this, then they may ask you to demonstrate that you have resided here, through a proof of address (e.g.: utility bills in your name showing consistent payments and usage in Portugal, bank statements from a Portuguese bank showing regular transactions). Entry and exit stamps in your passport can be also required.
