session question

in login page (page1):

if (txt_userName.Text == " asmaa" && txt_password.Text == "bbb")
{
    Session["user_name"] = txt_userName.Text;
    Response.Redirect("page2.aspx");
}

in page2:

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        PopulateDropDownList();
        lbl_session_userName.Text = "Welcome " + Session["user_name"].ToString();
    }
}

 how to check in every page in my project if the username can show a page