In general that would be pretty easy to identify. If the number of votes were large enough to impact an election you’d see voting numbers that are far greater than you expect based on the population and demographics of the area served by a particular voting office. In addition you’d see counts greater than you expect when certain people are working but not for others.

In addition usually you have to check in at a desk or table to get your ballot. An official dishonestly stuffing the ballot box would also have to somehow fabricate real voters checking in at the desk, or else there’d be more ballots than people who checked in and they’d identify the fraud. Where I live you check in with your ID card so unless the official had a bunch of IDs of valid registered voters they’d be caught.

Lastly voting fraud is a crime pretty much everywhere, so getting caught is bad.

A more realistic version of voting fraud is what is being planned in the US: getting supporters of a candidate (in this case Trump) to volunteer at voting locations and having those people fabricate evidence of fraud. This can just be their testimony, but it can be used later in lawsuits to give face value validity to accusations that the election was stolen, and used as justification for violence or a coup. This is what Trump tried, poorly, in 2020. They will try better in 2024.