Оформил: DeeCo
Автор: http://www.swissdelphicenter.ch

// Determine, if a Edit Field has password characters 
// Herausfinden, ob das Edit Feld Passwort Charakter hat 
function HasPasswordChar(AHandle: HWND): Boolean;
 var
   dwStyle: DWORD;
 begin
   dwStyle := GetWindowLong(AHandle, GWL_STYLE);
   Result := (dwStyle and ES_PASSWORD) = ES_PASSWORD;
 end;

 // Set password characters for the Edit Field 
// Passwort Charakter fur ein Edit Feld setzen 
procedure SetPasswordChar(AHandle: HWND; Value: Char);
 var
   S: String;
   len: Integer;
 begin
   len := Sendmessage(AHandle, WM_GETTEXTLENGTH, 0, 0);
   SetLength(S, len);
   SendMessage(AHandle, WM_GETTEXT, len+1, lparam(@S[1]));
   SendMessage(AHandle, EM_SETPASSWORDCHAR, Ord(Value), 0);
   SendMessage(AHandle, WM_SETTEXT, 0, Integer(PChar(S)));
 end;

 // Cancel the password characters and reveal the text 
// Password Charakter aufheben und den Text anzeigen 
procedure CancelPasswordChar(AHandle: HWND);
 var
   S: string;
   len: Integer;
 begin
   len := SendMessage(AHandle, WM_GETTEXTLENGTH, 0, 0);
   SetLength(S, len);
   SendMessage(AHandle, WM_GETTEXT, len + 1, lParam(@S[1]));
   SendMessage(AHandle, EM_SETPASSWORDCHAR, 0, 0);
   SendMessage(AHandle, WM_SETTEXT, 0, Integer(PChar(S)));
 end;

 // Example: 
procedure TForm1.Button1Click(Sender: TObject);
 var
   wnd: HWND;
 begin
   wnd := FindWindowEx(GetForeGroundWindow, 0, 'TEdit', nil);
   if wnd <> 0 then
     SetPasswordChar(wnd,'*');
 end;