In practice by the way the actual role of your appointed watchers is to figure out early whether you've won. They can see whether another candidate's ballots a

In practice by the way the actual role of your appointed watchers is to figure out early whether you've won. They can see whether another candidate's ballots are piling up faster than yours, they can estimate whether a table counting ballots for a district you're expected to dominate is being given way fewer ballots to count than you'd expected... Yes, they would obviously spot if some election worker is like adding a pile of pre-marked mass produced ballots to a pile or something, or if they were just putting half of your ballots in the wrong pile - but stuff like that basically never happens, whereas somebody will win and it'd be nice to know before it's announced if that's achievable.