How to check if user is authorized inside Action

I suggest first figuring out what kind of Authorization your using. ;)

The answer you posted is correct. From what I remember poking around the [Authorize] attribute and related ActionFilter code MVC internally calls Page.User.Identity.IsAuthenticated just like those code examples.

Request.IsAuthenticated should work for what you're trying to do.

If you just want to know if the user is logged in:

if (User.Identity.IsAuthenticated) { ... }

If you are trying to do anything role-specific:

if (User.IsInRole("Administrators")) { ... }

The User instance is a public property of the Controller class, so you always have access to it from a Controller you write. If no user is logged in you should have a GenericPrincipal for the User and a GenericIdentity for the User.Identity, so don't worry about checking for nulls.