This script reproduces Google Analytics' allowLinker plugin, resulting in a function that returns true if the linker parameter in the URL is valid and false otherwise. Uses the "old" linker plugin format. You can also pass a string as an argument to check if that string is a valid linker parameter.

allowlinker-ga.js

var checkLinker = function(str) {
  
  // First browser fingerprint method.
  // Uses the clientId / gid string, user agent, time, and browser plugin descriptions.
  var joiner = function (cidGid, offset) {
    var a = new Date, 
        b = window.navigator, 
        c = b.plugins || [];
    var d = [cidGid, b.userAgent, a.getTimezoneOffset(), a.getYear(), a.getDate(), a.getHours(), a.getMinutes() + offset];
    for (var e = 0; e < c.length; ++e) {
      d.push(c[e].description);
    }
    return jumble(d.join('.'));
  };
  
  // Second browser fingerprint method.
  // Uses the clientId / gid string, time, user agent, browser language.
  var joiner2 = function (cidGid, offset) {
    var a = new Date, 
        b = window.navigator,
        c = a.getHours() + Math.floor((a.getMinutes() + offset) / 60);
    return jumble([cidGid, b.userAgent, b.language || "", a.getTimezoneOffset(), a.getYear(), a.getDate() + Math.floor(c / 24), (24 + c) % 24, (60 + a.getMinutes() + offset) % 60].join("."));
  };
    
  // One-way hash of the fingerprint, included in the linker parameter.
  var jumble = function (arr) {
    var b = 1, c;
    if (arr) {
      for (b = 0, c = arr.length - 1; 0 <= c; c--) {
        var d = arr.charCodeAt(c);
        b = (b << 6 & 268435455) + d + (d << 14);
        d = b & 266338304;
        b = 0 != d ? b ^ d >> 21 : b
      }
    }
    return b.toString();
  };

  var linkerType, linker;
  
  // Check Linker validity and isolate the Linker parameter string.
  if (typeof str === 'string' && str.length) {
    if (!/_ga=/.test(str)) {
      return 'Invalid linker format in string argument!';
    }
    linker = str.split('&').filter(function(p) { return p.split('=')[0] === '_ga'; }).shift();
  } else {
    linkerType = /[?&]_ga=/.test(window.location.search) ? 'search' : /[#&]_ga=/.test(window.location.hash) ? 'hash' : undefined;
    linker = linkerType && window.location[linkerType].substring(1).split('&').filter(function(p) { return p.split('=')[0] === '_ga'; }).shift();
  }
  
  if (typeof linker === 'undefined' || !linker.length) {
    return 'Invalid linker format in URL!';
  }
  
  // Get the finger print and Client ID / Google ID strings from the parameter.
  var a = linker.indexOf('.'),
      b, c, d, fingerprint, cidGid;
  if (a > -1) {
    b = linker.substring(0, a);
    c = linker.substring(a + 1);
    d = c.indexOf(".");
    fingerprint = c.substring(0, d);
    cidGid = c.substring(d + 1);
  }
  
  // Jumble the Client ID / Google ID string and compare it against the fingerprint.
  // Check current minute, one minute back, and two minutes back.
  if (typeof cidGid !== 'undefined') {
    cidGid = cidGid.split('-').join('');
    return fingerprint === joiner(cidGid, 0) ||
      fingerprint === joiner(cidGid, -1) ||
      fingerprint === joiner(cidGid, -2) ||
      fingerprint === joiner2(cidGid, 0) ||
      fingerprint === joiner2(cidGid, -1) ||
      fingerprint === joiner2(cidGid, -2);
  }
}