determining URL that forwarded
If by forwarding you mean HTTP redirection, you can check the Referer header.
If you mean DNS resolving (e.g. distinguishing between your application being invoked via your own domain and .appspot.com one), there is SERVER_NAME environment variable (os.environ["SERVER_NAME"]) that stores the domain (e.g. www.example.com) used to issue the request.
I have a series of different domain names that I would like to all point (via URL forwarding from my domain host) to a google app engine application that reads what the forwarding URL is. So if the domain typed in was original XYZ.com, then when I am forwarded to my application, I can return what that original domain name was. I'm using the python variant. How best can I do this without coding for each and every variant?