Bad players don't use GTO because they don't know it. Good players play GTO to varying amounts. Imo many of them actually do it too much (GTO is only optimal against other strong players; it's not the best way to play vs. weak players).
If you know some GTO stuff you can usually recognize it pretty easily because it makes a lot of unintuitive plays, like overbetting on the Flop on some boards, betting 300% Pot on the River, or 4bet
bluffing with A5s. Most players never do any of this.