You can never "look" into the machine, all you can do is to put the machines under tamper-evident seals and trust the authority which distributes these.
I'm trying to think of a way that the machine could be designed where it's veracity is verified and viewable by all as it works. In other words there is some kind of mechanism that prevents falsifying the count. Something equivalent to being able to watch over people's shoulders as they count votes. A website that anyone can go to and monitor each machine and its code as it is working but with read-only ability.
Mechanical.... but still if one was crafty enough it still could be made to give a false impression....