Random Number Generator
Modern slot machines are computerized. The odds are programmed. In modern slot machines, the reels and lever exist only for historical and entertainment reasons. The positions of the reels depend on a Random Number Generator. There is a Random Number Generator in the machine's software.
The RNG is constantly generating random numbers at an extremely high rate. When a player presses the Play button the most recent random number determines the result. That means that the result depends on exactly when the game is played. The result would be different a fraction of a second earlier or later.
Some professional gamblers observe that the RNG does not actually generate random numbers. Indeed, most RNGs will eventually repeat their number sequence. Poor programming is its reason. Pseudo RNGs with very long periods are relatively easy to build. There is no computer that is able to complete a single period in the expected lifetime of the universe. It was Ronald Dale Harris, a former slot machine programmer, who knew the pseudo RNG code and seed values. Specific gambling games equations were discovered by him. Thanks to them the fact that the next set of selected numbers would be based on the previous games became known. But it is impossible for most machines, because numbers are picked by the RNG even when no one plays. So the numbers can't be exactly found in a sequence.

Payout Percentage
Slot machines usually pay out as winnings 82–98 per cent of the money that is wagered by players. This is known as the theoretical payout percentage. The minimum theoretical payout percentage varies among jurisdictions. Law or regulation are used to establish it. Nearly each casino has its own winning pattern. The amounts they pay and the frequencies of these pay-outs are reflected in it. The main criterion for selecting the winning patterns on slot machines is that they should yield a certain fraction of the money played to the house. The rest of the money goes to the players.
A slot machine's software is written and the theoretical payout percentage is set simultaneously. Changing the payout percentage after a slot machine has been placed on the gaming floor requires a physical swap of the software. Being a time-consuming process it is done not very often. There is sometimes a tamper-evident seal on the EPROM. To change it Gaming Control Board officials should be present. Other jurisdictions constantly ensure that slot machines contain only approved software.
The Nevada Gaming Commission is working now with Las Vegas casinos on technology that would allow the casino's slot manager to change the game, the odds, and the payouts remotely. Only after the selected machine has been idle for at least four minutes can the change be done. The machine must be locked to new players for four minutes and display an on-screen message informing potential players that a change is being made after the change is made.

Linked Machines
Linking the machines in a special way is made to offer a particularly large prize, or jackpot. Each slot machine in the group contributes a small amount to this progressive jackpot, awarded to a player who gets a specific combination of symbols. A single slot machine's jackpot is considerably less than the progressive jackpot.
There can also be multiple casinos consisting of multiple machines. In these cases the manufacturer owning these machines is responsible for paying the jackpot.

Near-miss Programming
The reel display of modern slot machines is under computer software's control. Combinations that are close to winning combinations can be displayed on the slot machine intentionally.
This practice is called near-miss programming.
There is a related phenomenon which is also called near-miss. The chance of a winning combination appearing on a pay line is controlled by the winning percentages programmed into the slot machine. The combinations above and below the payline are all roughly equally randomly distributed. There are roughly equally randomly distributed combinations above and below the pay line. The occurance of this phenomenon is possible when abbreviated physical reels are used to display a win pattern based upon the RNG. In video slot machines, virtual reels are used and the symbols that appear around the winning line are usually an accurate depiction of how the reels were mathematically modeled.
The issue of a near-miss above or below the pay line was also the subject of the Nevada Gaming Commission investigation. They ruled that this was legal, so long as the near-miss above or below the pay line was not specially programmed. In other words, any other combination must be just as likely to occur as the near-miss. The machine cannot be specially programmed to show winning combinations more frequently than other combinations above or below the pay line.
Australia also forbids to use near-miss programming, where a near miss is inaccurately displayed. Stop motion cameras are used by regulators for manufacturer's practices audit.

Slot Machines Fraud
Sometimes mechanical slot machines and their coin acceptors seem to belong to cheating devices and other scams.
Coin acceptors have become obsolete in favor of bill acceptors and modern slot machines are controlled by EPROM computer chips. These machines and their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures and are difficult to defraud. Nowadays microwaves are used to defraud slot machines.

I tis important to now what you are playing.
Always make sure that you insert the correct amount of coins into a machine. Always cash out your credits before leaving a machine.
Only if you have fun your gambling won't be stressful. Use money management. Increase your bets when you are winning, and decrease your bets when you are losing.

You can be sure that slot machines are designed to be honest. What you can really doubt is the honesty of some manufactures.
When the first slots only appeared cheating was impossible because they worked mechanically, with independent wheels spinning and then stopping randomly. Today’s slot machines are light years away from those lumbering machines of old. These machines contain computer chips that are programmed to select numbers randomly. The selected number determines the symbol which then comes out. There is a computer code. It generates the random numbers and determines whether the slot machine is loose or tight. The gaming inspector will examine the machine to make sure it will not deceive the player. There is a legal rate according to which the machine has to pay out. Of course it can pay lower, which happen very seldom.
It is next to impossible to say whether the chip is programmed not in a right way because the machine is not supposed to pay out with every spin. The machines have to be examined before people begin to play them. Of course it is very difficult to examine every single machine, that’s why inspectors examine only brands. In the case of the approval of the brand the manufactures can produce the slot machines, even change them if it doesn’t go out legal bounds. Some dishonest manufactures use the right to produce machines, but use a different chip, which is programmed to pay less or not so often.
For example, the Nevada Gaming Commission caught American Coin using chips that did not allow poker royal flushes. Similarly, Universal Distributing Company was caught programming machines to show a “near miss” of a jackpot. The effect was two-fold: the machine didn’t pay off, and customers continued to play the machine, thinking that they were very close to a big jackpot. If you think you’re playing a machine that is not honest, you can complain. In the case if the complaint the inspectors will visir the casino and check whether the machines have a legal chip. Manufacturers that cheat are fined or they can lose their license.