laravel/app/Http/Filters/GuestFilter.php

<?php namespace App\Http\Filters;

use Illuminate\Contracts\Auth\Authenticator;
use Illuminate\Http\RedirectResponse;

class GuestFilter {

	/**
	 * The authenticator implementation.
	 *
	 * @var Authenticator
	 */
	protected $auth;

	/**
	 * Create a new filter instance.
	 *
	 * @param  Authenticator  $auth
	 * @return void
	 */
	public function __construct(Authenticator $auth)
	{
		$this->auth = $auth;
	}

	/**
	 * Run the request filter.
	 *
	 * @return mixed
	 */
	public function filter()
	{
		if ($this->auth->check())
		{
			return new RedirectResponse(url('/'));
		}
	}

}