Our county used numbered "seals" on the ballot boxes, and I instructed my workers to check the numbers when the box was sealed at the polling place and again upon arrival at the County Courthouse to be certain that the seal had not been removed. Each election judge had about 6 seals to choose from (all with different numbers) and we also checked to see that there were no duplicate numbers. You could not removed the seals without breaking them.
Even with these precautions and elaborate instructions, I watched when ballot boxes were dropped in the parking lot and spilled their contents all over. Obviously they had left their polling places unsealed, despite all the regulations.
I haven't done this in a few years, and the system here has changed. We have "scanner" ballot boxes that count the votes as we cast them now. I haven't figured out how you could tamper with them, so I haven't figured out what you need to watch for.
Generally the city-hired poll workers were very uncooperative. They considered our efforts to "monitor" the election to be insulting to them -- despite all State Statutes to the contrary.