By having dozens of people watching everything except the actual marking of the ballot. Scrutineers are highly motivated: they want to catch the other candidate cheating.

I've been a scrutineer in a polling room. I could watch and challenge anything except the marking of a ballot. And I did. So did other scrutineers. The odds of the mobster keeping a rotating scheme without detection approach zero with great rapidity, especially since staying in the ballot after casting your vote is not permitted.

Yup. In the U.S. they are generally called poll watchers.

https://www-m.cnn.com/2016/10/29/politics/poll-watchers-moni...

Also, a voter who is being intimidated by a mobster could just walk up to a poll worker or police officer and report it.

He wouldn't stay in the ballot - he'd pocket the slip and take it outside...

Returning to my point, which is that he'd need to do this without anyone noticing.

A bit difficult since there are officials watching you put the ballot into the box. Or, gee, that's weird, failing to put the ballot into the box.

Couldn't they put a random piece of paper in the ballot box?

What happens if a white piece of paper is found? Do all the votes in the box get cancelled?