How Can I Check Table Before I Insert Data By C#.

protected void btnRegister_Click(object sender, EventArgs e)
        {
            string connetionString = "Data Source=qweq;Initial Catalog=Test_DB;User ID=asd;Password=asdsf";
            SqlConnection conn = new
            SqlConnection(connetionString);
            string commText = "insert into login(User_Name,Password,Address,City,State,PIN,Phone_no,Email_id,Created_By,Created_on) values('" + txtUserName.Text + "','" + txtPassword.Text + "','" + txtAddress.Text + "','" + txtCity.Text + "','" + txtState.Text + "','" + Convert.ToDouble(txtPin.Text) + "','" + Convert.ToDouble(txtPhoneNo.Text) + "','" + txtEmailId.Text +  "','" + "Alok" + "','" + DateTime.Now + "')";
            SqlCommand cmd = new 
            SqlCommand(commText, conn);
            conn.Open();
            int no = cmd.ExecuteNonQuery();
            conn.Close();
            if(no>0)
            {
                Label1.Text= "User register register sucessfully";
            }
            else
            {
                Label1.Text = "Error";
            }
            txtUserName.Text = String.Empty;
            txtPassword.Text = String.Empty;
            txtAddress.Text = String.Empty;
            txtCity.Text = String.Empty;
            txtState.Text = String.Empty;
            txtPin.Text = String.Empty;
            txtPhoneNo.Text = String.Empty;
            txtEmailId.Text = String.Empty;

        }