<script type="text/javascript">
  $(function() {
    $("#empcode").change(checkEmpCode);
  });

  function checkEmpCode() {
    $.ajax({
      type: "POST",
      url: "Service.asmx/CheckEmpCode",
      data: "{empcode: '" + $('#empcode').val() + "'}",
      contentType: "application/json; charset=utf-8",
      dataType: "json",
      success: function(response) {
        if (response.d != "0") {
          $("#failureimage").show();
        }
        else{
          $("#successimage").show();
        }

      }
    });
  }

</script>

[WebMethod]
public int CheckEmpCode(string empcode)
{
  string connect = @"Server=SERVER;Database=Database;Trusted_Connection=True;";
  string query = "SELECT COUNT(*) FROM Employee WHERE empcode = @empcode";
  using(SqlConnection conn = new SqlConnection(connect))
  {
    using(SqlCommand cmd = new SqlCommand(query, conn))
    {
      cmd.Parameters.AddWithValue("empcode", empcode);
      conn.Open();
      return (int)cmd.ExecuteScalar();
    }
  }
}