Random Number Generator
Nowadays slot machines are computerized. They programme the odds. 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. The machine's software contains it.
The RNG is constantly generating random numbers at an extremely high rate. As soon as the Play button is pressed, the most recent random number is used to determine the result. The result is different depending on exactly when the game is played. The result is different every fraction of a second.
It was noticed that the RNG does not actually generate random numbers. Indeed, most RNGs will eventually repeat their number sequence. The cause of it is in poor programming. Pseudo RNGs with very long periods are relatively easy to build. A single period can't be completed by any computer in the expected lifetime of the universe. The access to the pseudo RNG code and seed values was known to Ronald Dale Harris, a former slot machine programmer. Specific gambling games equations were discovered by him. They allowed to predict that the next set of selected numbers would be based on the previous games played. However even if nobody uses the machine the RNG continues to pick numbers. So you can't find an exact place of a number in the sequence.

Payout Percentage
Slot machines are typically programmed to pay out as winnings 82–98 per cent of the money that is wagered by players. The term theoretical payout percentage is used to denote it. Jurisdictions serve to control the minimum theoretical payout percentage. It is typically established by law or regulation. A certain winning pattern exists nearly in every casino. The amounts they pay and the frequencies of these pay-outs are reflected in it. The winning patterns on slot machines are carefully selected to yield a certain fraction of the money played to the house. The rest of the money is given back to the players.
At the factory when the software is written a slot machine's theoretical payout percentage is set. Changing the payout percentage requires a physical swap of the software. Based on current technology, this is a time-consuming process and as such is done infrequently. One can find a tamper-evident seal on the EPROM in certain jurisdictions. Gaming Control Board officials should be called for changing it. Other jurisdictions constantly ensure that slot machines contain only approved software.
Thanks to the newly developed technology the game, the odds, and the payouts would be able to be changed by the casino's slot manager remotely. Only after the selected machine has been idle for at least four minutes can the change be done. After the change is made, 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.

Linked Machines
Often machines are linked together in a way that allows a group of machines 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. The amount of the progressive jackpot is far higher than any single slot machine's.
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
The reel display of modern slot machines is controlled by computer software. That is why it is possible to make the slot machine frequently display combinations that are close to winning combinations.
Such showing combinations that are similar to winning combinations more frequently than would occur randomly is called near-miss programming.
This term is also used for a related phenomenon. Winning percentages programmed into the slot machine control the chance of a winning combination appearing on a pay line. However, the combinations appearing above and below the pay line are all roughly equally randomly distributed. There are roughly equally randomly distributed combinations above and below the pay line. Only if abbreviated physical reels are used to display a win pattern based upon the RNG this can occur. The symbols that appear around the winning line in video slots are an accurate depiction of how the reels were mathematically modeled.
There was an investigation held by the Nevada Gaming Commission concerning the issue of a near-miss above or below the pay line. If the near-miss above or below the pay line was not specially programmed it was stated to be legal. In other words, the near-miss must be just as likely to occur as any other combination. The machine can't 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. Due to this, regulators use stop motion cameras to audit manufacturer's practices.

Slots Fraud
Mechanical slot machines and their coin acceptors were sometimes susceptible to cheating devices and other scams.
EPROM computer chips controls modern slot machines and coin acceptors have become obsolete in favor of bill acceptors. They are designed with advanced anti-cheating and anti-counterfeiting measures and are difficult to defraud. One of the recent attempts at defrauding slot machines is directing microwaves toward it to disrupt its proper functioning.

There are a lot of myths and superstitions about slots.
You can spot a winning slot machine.
The only way you'll spot a winning slot machine is when the bells are ringing and the money is dropping. RNG is the main element of every slot machine and one never knows what combination it will form.
Casinos place loose slots near the cashier, aisle ways, etc.
In reality it is not to casino’s advantage to place loose slots at one and the same place, because in this case gamblers would play only these slots. The slots are mixed and nobody can say for sure whether this slot is a loose one or not.
Count the number of symbols on the reels to determine your odds.
This is mathematically impossible, remember the RNG (random number generator) determines the stops. Thereare literally millions of combinations on a 3 reel slot machine, so there is no way of determining odds by the symbols on the reels alone.
Casinos can tighten or loosen slot machines from the back office.
It is not as simple as that. The casino can’t just change the chip of the slot machine. It is illegal and it has to be permitted by the commission.
A slot machine that has not been paying is due to hit.
It is not so. With every game the chances of winning are the same.
Pulling the slot machine handle produces more wins than pressing the spin button and visa versa.
It is also a mistake. These things are only mechanisms for beginning the game. The machine knows whether you win or lose before the symbols begin to stop.
Playing with hot coins improves your chances.
It is false. Nothing can affect the outcome, the machine doesn’t care and doesn’t know whether your coins are cold or hot.
Casinos set slot machines to payback more on the weekends.
One more rude mistake. The chances and odds are always the same. It doesn’t matter when you play.

It is true that slots are designed to be honest and not to deceive a player. However, in the past certain manufacturers have been caught using computer chips that cheat the customer.
In the old days, slot machines worked mechanically, with independent wheels spinning until they each stopped randomly. Today’s slot machines are light years away from those lumbering machines of old. Now the integral part of any slot machine is a chip, which is 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. That is, inspectors make sure that the machine pays off at the legal rate or better.
Because slot machines don’t pay off with every spin, it’s difficult for customers to tell whether a slot machine’s chip has been programmed to pay off less often than is legally required. Before the machines are installed in casinos they are examined by inspectors. But you must understand that inspectors can’t examine every machine. They examine the brands. If inspectors approve the brand, they can produce machines, and change them. But these changes must cover legal requirements. Manufacturers can change chips to make a machine pay less as long as the inspectors have approved that particular chip.
There is an example when there was inserted an illegal tip which doesn’t allow 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. Inspectors will respond by pulling the slot machine’s chip and seeing if it is an approved chip. Manufacturers that cheat are fined or they can lose their license.