If the Mavic is brand new, DJI GO will prompt you to register it when you connect DJI GO to the Mavic. If you don't see the prompt to register it, that means someone else already registered it. Beyond that, you could check the batteries in DJI GO to see how many times they've been charged or check if the Mavic's internal memory card contains any DAT flight log files (like this) that were created prior to when you received the Mavic.