1 simple way to know if you are being invaded.

The game does not pause during the map or online contracts menu.

You're unable to be hacked if you are searching for a game online.

Another way to tell if someone is hacking you is no online, crime, convoy, etc, notifications will popup, and you will be unable to start any missions or join a gambling game like the shell game or chess for example.

(Please be free to tell me if I am missing anything and/or if any of this is wrong. I'm sure it is not wrong though.)

People who pointed out any of my mistakes, thank you for doing so. It allows me to make the guide as best as I can.