.net Forms Authentication

Question

Hi, I am currently implementing forms authentication and wanted to know how I can use a sessions to check if the user is logged in, then redirect the user to the login page or the requested page depending on the session. I know that the user is always directed to the login url first in forms authentication, so I wanted to check the session so the user wouldn't have to sign in again if they are already signed in or want to open a second window. thanks