The RNG
There are many modern computerized slot machines. So the odds are whatever they are programmed to be. In modern slot machines, the reels and lever exist only for historical and entertainment reasons. The positions of the reels are chosen by a Random Number Generator. There is a Random Number Generator in the machine's software.
Random numbers are constantly generated by the RNG at an extremely high rate. When you pres the Play button the result is determined by the most recent random number. The result varies depending 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. Most RNGs will repeat their number sequence. This behavior is due to poor programming. It is relatively easy to build pseudo RNGs with very long periods. A single period can't be completed by any computer in the expected lifetime of the universe. Ronald Dale Harris, a former slot machine programmer, was the only one who had access to the pseudo RNG code and seed values. It was he who discovered equations for specific gambling games like Keno. The fact that the next set of selected numbers would be based on the previous games played was revealed. However, this is impossible for most machines, because the RNG picks numbers even when the machine is not being played. So you can't find an exact place of a number in the sequence.

Percentage Of The Payout
The percentage of winnings paid out of the money that is wagered by players is 82–98 per cent. This payout percentage is called theoretical. The minimum theoretical payout percentage is controlled by jurisdictions. It is typically established by law or regulation. A certain winning pattern is determined by every casino individually. It concerns the amounts they pay and the frequencies of these pay-outs. The selection of the slot machines winning patterns is chosen in such a way that it could yield a certain fraction of the money played to the house. The players get the rest of the money.
A slot machine's theoretical payout percentage is set at the factory when the software is written. Changing the payout percentage requires a physical swap of the software. As it is a time-consuming process it is done infrequently. One can find a tamper-evident seal on the EPROM in certain jurisdictions. It can only be changed in the presence of Gaming Control Board officials. According to other jurisdictions slot machines are randomly audited to ensure that they contain only approved software.
The technology being developed by the Nevada Gaming Commission would allow the casino's slot manager to change the game, the odds, and the payouts remotely. The change can be done only after the selected machine has been idle for at least four minutes. 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
Offering of a particularly large prize, or jackpot becomes possible when a group of machines is linked in a special way. A small amount is contributed by each machine in the group to this progressive jackpot which is given to a player who gets a specific combination of symbols. A single slot machine's jackpot is considerably less than the progressive jackpot.
In some cases multiple machines are linked across multiple casinos. In these cases, the machines may be owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss Programming
Computer software controls the reel display of modern slot machines. 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.
There is a related phenomenon that is also sometimes called near-miss. Winning percentages programmed into the slot machine control the chance of a winning combination appearing on a pay line. 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. This occurs if abbreviated physical reels are used to display a win pattern based upon the RNG. In video slot machines the symbols that appear around the winning line are usually 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. So long as the near-miss above or below the pay line was not specially programmed it was recognized legal. In other words, the near-miss must be just as likely to occur as any other combination. It is impossible to program the machine to show winning combinations more frequently than other combinations above or below the pay line.
In Australia near-miss programming, where a near miss is inaccurately displayed is not allowed either. To audit manufacturer's practices regulators use stop motion cameras.

Slot Machines Fraud
Sometimes mechanical slot machines and their coin acceptors seem to belong to cheating devices and other scams.
Modern slot machines are controlled by EPROM computer chips and coin acceptors have become obsolete in favor of bill acceptors. Different advanced anti-cheating and anti-counterfeiting measures have been taken to make it difficult to defraud these machines. Recent attempts at defrauding slot machines involve manipulating the EPROM, such as by directing microwaves toward it to disrupt its proper functioning.

Here is the list of the most amusing myths concerning slots.
You can spot a winning slot machine.
The only way one can say that theslot machine is a winning one is only to catch this moment, when the machine pays out. 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.
If this were true, it would be the only machines anyone would play. A casino mixes slots all over the casino floor. No one with any experience at playing slots would make a statement like this.
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.
Again false. It makes no difference, as the computer only uses these as mechanisms to start the game. Only the computer determines a win or a loss and it has already determined if you are a winner or loser even before the first reel stops spinning.
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.
Wrong again. Your odds of winning are the same no matter if it's 4:00pm Monday, 8:00pm Saturday or 3:00am Wednesday. Casinos can not and do not do this.

Slot machines are honest because state gaming commissions make sure they are. What you can really doubt is the honesty of some manufactures.
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 numbers the computer selects correspond to the symbols that appear on the slot machine’s screen. 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. Inspectors look at brands of machines and their chips before they are installed in casinos, but they can’t inspect each machine. Once inspectors approve a brand of machine, the manufacturers can make changes to individual machines as long as the changes fall within the legal bounds. Manufacturers can change chips to make a machine pay less as long as the inspectors have approved that particular chip.
In one of the casinos the chip was programmed in such a way that royal flushes would never come out. Or a chip that shows near misses of a jackpot. One the one hand the machine would never have a jackpot and the player would keep on playing. Don’t keep silent if you come across such slot machine. 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.