For those machines counting paper ballots, before using at polling places, they need to run a certain number of dummy ballots with the results known through the machines, compare the results, then lock the machines if they are accurate. That could and should be done at each polling place where the number of machines is relatively small. Machines that fail are taken out of service. It shouldn't be difficult if you have adequate and honest poll watchers.
Poll watchers are not allowed to DO anything at the polls. They are not allowed to interfere in any way. Even election workers are not allowed to change the procedures, and I’m not sure how much leeway the Chief inspector has. Everything hinges on the number of ballots run matching the number of signatures in the book.