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 depend on a Random Number Generator. It is included into the machine's software.
The rate of generating random numbers by the RNG is extremely high. When you pres the Play button the result is determined by the most recent random number. The result is different depending on exactly when the game is played. A fraction of a second earlier or later, and the result would be different.
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. It is relatively easy to build pseudo RNGs with very long periods. No computer can complete a single period in the expected lifetime of the universe. Ronald Dale Harris, a former slot machine programmer, had access to the pseudo RNG code and seed values. It was he who discovered equations for specific gambling games like Keno. Thanks to them the fact that the next set of selected numbers would be based on the previous games became known. But the RNG picks numbers even when the machine is not being played. So the player cannot tell where in the sequence they are.

Percentage Of The Payout
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. Jurisdictions serve to control the minimum theoretical payout percentage. Law or regulation are used to establish it. There is a certain winning pattern nearly in every casino. It contains the information about 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 rest of the money is given back to the players.
A slot machine's theoretical payout percentage is set at the factory when the software is written. Changing the payout percentage after a slot machine has been placed on the gaming floor requires a physical swap of the software. Based on current technology, this is a time-consuming process and as such is done infrequently. There is sometimes a tamper-evident seal on the EPROM. Only Gaming Control Board officials can change it. Other jurisdictions randomly audit slot machines 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. 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.

Machines Linking
Often machines are linked together in a way that allows a group of machines to offer a particularly large prize, or jackpot. If a player gets a specific combination of symbols the progressive jackpot from this group of machines is given to him. The amount of the progressive jackpot is far higher than any single slot machine's.
Sometimes multiple machines form multiple casinos. In these cases, the machines may be owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss Programming
The reel display of modern slot machines is under computer software's control. That is why it is possible to make the slot machine frequently display combinations that are close to winning combinations.
This practice is called near-miss programming.
A related phenomenon is sometimes 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. 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. In video slot machines 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. If the near-miss above or below the pay line was not specially programmed it was stated to be legal. In other words, any other combination must be just as likely to occur as the near-miss. Winning combinations aren't supposed to be shown more frequently than other combinations above or below the pay line.
In Australia the usage of near-miss programming, where a near miss is inaccurately displayed is also illegal. To audit manufacturer's practices regulators use stop motion cameras.

Slots Fraud
Mechanical slot machines and their coin acceptors were sometimes considered to be cheating devices and other scams.
Modern slot machines are controlled by EPROM computer chips and coin acceptors were changed for bill acceptors. It is difficult to defraud these machines because they their bill acceptors are designed with advanced anti-cheating and anti-counterfeiting measures. An example of modern ways of slot machine defrauding is directing microwaves toward it to disrupt its proper functioning.

To know what you are playing is necessary for you.
Always make sure that you insert the correct amount of coins into a machine. Always cash out your credits before leaving a machine.
Slots shouldn't be a stress for you, just enjoy. 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. 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. Modern slot machines differ form the old ones greatly. 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. The machine has to pay out either to the legal rate or better. Of course the latter happens 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. Using their right to produce slot machines they insert different chips, which makes the machine pay less.
In one of the casinos the chip was programmed in such a way that royal flushes would never come out. Similarly, Universal Distributing Company was caught programming machines to show a “near miss” of a jackpot. The player would never hit a jackpot but keep on playing maximum bets. 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.