Authentication is usually handled by web server itself and you, as a user (developer or regular one) should be either granted or denied access.
You should be able to see login/password credentials in the request. For Basic Authentication they are passed in the request header, for SOAP, depending on the implementation, they can be passed in the Header section of SOAP Envelope (passed in the body of request).
Depending on the implementation, you might be able to get an access to the login credentials on the server side in tested application's code, but I am not sure why you would need this for tests.
Could you provide more details about your test and what you'd like to be able to do?