How to enable single sign on option in WP8?‏‏

You can save the User Login information to IsolatedStorage. Each user has unique name, I mean there is no any duplication in UserName. So with the help of UserName you can store the information to IsolatedStorage.

For e.g.

public void SaveToStorage(string UserName)
{
    if (IsolatedStorageSettings.ApplicationSettings.Contains(UserName))
    {
        return;
    }
    IsolatedStorageSettings.ApplicationSettings["LoginStatus"] = UserName;
    IsolatedStorageSettings.ApplicationSettings.Save();
}

public bool GetFromStorage()
{
    if (IsolatedStorageSettings.ApplicationSettings.Contains(LoginStatus))
    {
        return true;                     // User is already Login
    }
    return false;
}

With the help of above two function you can check this on Application Activated or Launching event whether user is already login or not.