ga_block_check.js

if (window.fetch)
    fetch('https://www.google-analytics.com/collect').then(function(response) {
        if (!response.ok) {
            throw Error(response.statusText);
        }
        return response;
    }).then(function(response) {}).catch(function(error) {
        window.dataLayer.push({
            'event': 'ga-blocked'
        });
    });