I'm not sure, actually. An interesting question! Thanks for bringing it up :)

Let me introduce a unique custom that occurs on election days in Japan. At each polling station, the voter who get there the earliest on the day is asked to look inside of the ballot boxes to make sure they are empty before voting. When I learned [or "While I knew...] that this custom was meant to guarantee the fairness of elections, I thought it was probably an imitation of a system from somewhere in Europe or the United States, but according to my English teacher from the United Kingdom, the UK has no such custom. Does your country have an equivalent custom? As far as I know, there's no such equivalent custom in Canada. But I will note that in Canada, elections are conducted by a non-partisan agency (called Elections Canada), and as well as the people from that agency, there are candidate representatives (if they choose to send them) present to oversee the fairness of the process. So I don't think there is much concern here for monkey business. Probably the election officers and the candidate representatives themselves inspect the ballot box before poll open.

We would expect the people in charge of voting to certify the fairness, not the the individual voters. (US citizen here)