302 wont work. It simply redirects to the destination page, and then the onload would be triggered. What you need to find is something which generates an error code, ie 4xx/500. When logged in, this generates a 406, because I set "format" to something invalid:

https://twitter.com/account/use_phx?setting=false&format...

But when logged out, it redirects to the login form which ultimately provides a 200 status code.

There are probably several other ways of making twitter generate a HTTP error code.