Check ReturnUrl Is Valid Before Redirecting?

I'm using ASP.NET Membership and Form Authentication and before redirecting to the returnURL I wanted to validate it. For those unfamiliar with the workflow, basically if you request a page that requires that you are authenticated, you are redirected to a login page. In the URL string you'll see a parameter called returnURL, e.g. [URL]

Whether you use this in a redirect such as Response.Redirect(returnURL) or indirectly through the FormsAuthentication.RedirectFromLoginPage method, it passes without validating returnURL.

FormsAuthentication.RedirectFromLoginPage does have a security check that it is isn't leaving the domain, but that still doesn't stop someone from putting enough random characters to cause an error.

I tried using System.IO.File.Exists(Server.MapPath(returnURL)) but given enough illegal characters it cause Server.MapPath to error.

Note: URLEncoding doesn't work because we are not cleaning a parameter, but the primary URL.