How to know if anyone else hack my iPhone?

they can't, but if you fear your appleID is compromised then read

If you think your Apple ID has been compromised - Apple Support (OM)

When I change my Apple ID, it shows a map that indicates a device in Washington DC. I live in NJ! How do I know if I am hacked?

Every month my Apple ID changes and I have to change it again on all my devices (Ipad, Ipod, iPhone 12 and a desktop computer).