Check if user has permission to access a url (for use with logon return url)

When a user logs on, I check if there is a return url, and if so I redirect to it, as per the MVC application template.

if (!String.IsNullOrEmpty(returnUrl))
{          
    return Redirect(returnUrl);
}

How would I check if this URL was valid (routes to a controller/action), and the user has permission to access it? My actions are all protected using the standard authorize attribute.

Thanks for your help,

Stew

Copyright Notice：Content Author:「stew」，Reproduced under the CC 4.0 BY-SA copyright license with a link to the original source and this disclaimer.
Link to original article：https://stackoverflow.com/questions/1807604/check-if-user-has-permission-to-access-a-url-for-use-with-logon-return-url