How To Add validation( check if value exists in database) in this code

SqlConnection sqlcon = new SqlConnection(@"Server=ADMIN-PC\SQLEXPRESS;Database=ELMANAR; Integrated Security = True");
        SqlDataAdapter da;
        DataTable dt1 = new DataTable();

        Data_Access_Layer.DataAccessLayer DAL = new Data_Access_Layer.DataAccessLayer();
        SqlCommand cmd = new SqlCommand();




        private void btnsave_Click(object sender, EventArgs e)
        {

            try
            {
              
                cmd.CommandType = CommandType.Text;
                cmd.Connection = sqlcon;

                cmd.CommandText = "insert into TBL_CATEGORYCUSTOMERS (catcustomer_id,catcustomer_name,catcustomer_note) values ('" + txtid.Text + "','" + txtname.Text + "','" + txtnote.Text + "')";
                sqlcon.Open();
                cmd.ExecuteNonQuery();
              
                    
                    MessageBox.Show("Saved");
                    sqlcon.Close();
                    dt1.Clear();
                    da.Fill(dt1);

                    btnsave.Enabled = false;
                    btnadd.Enabled = true;
                    txtid.Clear();
                    txtname.Clear();
                    txtnote.Clear();
                                    
            }
       

            catch { }