Google Tag Manager

You’ve to check the opt-out cookie in your GTM (Google Tag Manager). If this cookie is set, you shouldn’t fire the Google Analytics code.

Example:
If your UA-Code is UA-12345-6, the cookie name will be ga-disable-UA-12345-6 with the value true.

You also have the possibility to check for the window property, instead of the cookie. This is only available, if the user clicked on the opt-out link and has the value “true“.
console.log(window["ga-disable-UA-12345-6"]);