Google provides an excellent tool for maintaining the security of your GISD Google account. It's called the Security Checkup tool.
Some scenarios where you should use this tool are:
1) You get an email notification saying something like "Your email address - email@example.com - was used to login to App." If you don't recognize the app, use the Security Checkup tool to remove access to that app.
2) You get an email notification saying "New device signed into firstname.lastname@example.org" If you did not sign-in on a new device, go to the Security Checkup Tool to remove that device.