When looking on a GitHub repository, this script checks if you already have a fork of it on your account.

fork-checker.user.js

// ==UserScript==
// @name         Fork-Checker
// @namespace    https://github.com/Koopzington
// @version      0.1
// @description  When looking on a GitHub repository, this script checks if you already have a fork of it on your account.
// @author       [email protected]
// @match        https://github.com/*/*
// @grant        GM_xmlhttpRequest
// ==/UserScript==

(function() {
    'use strict';

    // First we need to check if the current page actually IS a repository since there are other pages matching the URL format
    var pagecheck = document.getElementsByClassName('repohead');
    if (pagecheck.length > 0) {
        // Grab page title
        var repoName = document.getElementsByTagName('title')[0].innerHTML;
        // Strip the part behind ':'. We've got vendorname/reponame now
        repoName = repoName.split(':')[0];
        var originalName = repoName;
        // We only want the reponame
        repoName = repoName.split('/')[1];
        // Grab username
        var username = document.getElementsByName('user-login')[0].getAttribute('content');
        // Check if current repository page isn't actually one of yours.
        if (username + "/" + repoName !== originalName) {
            var outputTarget = document.getElementsByClassName('pagehead-actions')[0];
            var content = document.createElement('button');
            content.className = 'btn btn-sm';
            GM_xmlhttpRequest({
                method: "GET",
                url: "https://github.com/" + username + "/" + repoName,
                onload: function(responseDetails) {
                    console.log(responseDetails);
                    if (responseDetails.status == 200) {
                        content.className += ' btn-primary';
                        content.innerHTML = 'Already forked!';
                    } else if (responseDetails.status == 404) {
                        content.innerHTML = 'Not forked!';
                    }
                    // Add fancy button with result next to the fork button
                    outputTarget.appendChild(content);
                }
            });
        }
    }
})();