Physically, assuming that you are refering to two identical slot machines (whose only difference is that one pays more than the other,) there is no physical difference whatsoever. The only difference is that the computer inside one slot machine is telling it to give more winning combinations than another machine. Basically, they are just programed different but are not PHYSICALLY different. It will help to understand a little more about the way slot machines work. The "reels" you see spinning in front of you have nothing to do with determining wether or not you will win. The reels just show you what combination the RNG (the Random Number Generator) inside the machine came up with. The slot machine manufacturers all the time try to eliminate the reels altogether, by replacing them with a computer screen (which is much cheaper to manufacture/install/maintain,) but customers hate them. Pycholigically, the consumer feels like they are getting a "fair shake" with the reels, whereas they feel like they are getting cheating when they play a machine that shows a computer-generated image of the reels on a monitor- even though there is absolutely no difference in the actual outcome.

In Australia, Slot machines (We call them Pokies, usualy - Short for "Poker Machines") legaly have to be set to return 84 to 92 Percent(Depending on the individual machine's settings) of what they take - though not set to do so for an individual player, but for overall takings. This does not stop one from winning jackpots, however, as this is governed by the Random result generator inside the machine.

Nothing pysically different. They just have different payout tables.