Checking if a user is logged in

Hi i have the following code, which allows a user to login, my question is how do i go about checking a user is logged in, and will i have to do it on each page that requires a log in;

  public bool CheckUserCredentials(string username, string password)
{
try
{

string UserPassword = FormsAuthentication.HashPasswordForStoringInConfigFile
("P283NC9G" + this._Password, "SHA1");
SqlCommand command = new SqlCommand("Users_CheckUser", conn);
command.Parameters.Add("@UserName", SqlDbType.VarChar).Value = this. ...