Use one of the string to number function inside a try except block, and test for an exception. Example:

function IsStrANumber(NumStr : string) : bool;
begin
  result := true;
  try
    StrToInt(NumStr);
  except
    result := false;
  end;
end;
procedure TForm .Button Click(Sender: TObject);
begin
  if IsStrANumber(Edit .Text) then
    ShowMessage(Edit .Text) else
    ShowMessage('Not a Number');
end;