procedure TForm1.Button1Click(Sender: TObject);
var I, Code : Integer;
begin
    Val(Edit1.Text, I, Code);
    if Code <> 0 then
     begin
         ShowMessage('Not a number!');
     end
    else
     begin
         Canvas.TextOut(10, 10, 'The number = ' + IntToStr(I));
     end;
end;
Testing for a number input and not a letter from the alphabet.
See Val in the Delphi Help files.