[lmikle]

Через OnKeyPress эдита.

Код:

procedure TForm1.Edit1KeyPress(Sender : TObject; var Key : Char);
begin
  If Not (Key in ['0'..'9','.']) Then Key := #0 Else
    If (Key = '.') And (Pos('.',Edit1.Text) > 0) Then Key := #0;
end;