procedure TForm1.Button1Click(Sender: TObject);
begin
Memo1.Perform(EM_UNDO, 0, 0);
Memo1.Perform(EM_EMPTYUNDOBUFFER, 0, 0);
Button1.Enabled:=Memo1.Perform(EM_CANUNDO, 0, 0)<>0;
end;

procedure TForm1.Memo1Change(Sender: TObject);
begin
Button1.Enabled:=Memo1.Perform(EM_CANUNDO, 0, 0)<>0;
end;