I heard rumors about testing of machines that could have a bunch of things change via a central control. I don't believe most machines actually do that though. What I was hearing about was machines that could literally change everything, including the available games and the appearance via video screens. They did have rules during the testing period. Something about no changes while someone is playing.
http://www.nytimes.com/2006/04/12/t...-a-casino-can-change-the-slot-machine-in.html
I know in Nevada the Gaming Commission can order a machine be pulled out at any time and tested for payouts. They hire expert programmers who can figure out how the pay tables are set up and calculate the average theoretical payout. They also approve slot machine programming, and they're not supposed to be allowed to use an unapproved program where the Gaming Commission hasn't signed off on it. Certainly IGT, WMS, or Aristorat all claim that they design their machines to be fair, because they wouldn't otherwise be allowed to sell them. There might not be this level of scrutiny on a cruise ship though, or at tribal casinos.