How to validate session?it should identify new user and current user?

protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            if (Session["User"] == null)
            {
                Response.Redirect("login.aspx");
            }
        }
        if (Session["SessionId"] == null)
        {
            Response.Redirect("login.aspx");
        }
        Response.Buffer = true;
        Response.ExpiresAbsolute = DateTime.Now.AddDays(-1d);
        Response.Expires = -1500;
        Response.CacheControl = "no-cache";
    }