Random Number Generator
Nowadays slot machines are computerized. So the odds are whatever they are programmed to be. The reels and lever are present for historical and entertainment reasons only. The positions of the reels are chosen by a Random Number Generator. The machine's software includes a Random Number Generator.
The rate of generating random numbers by the RNG is extremely high. The most recent random number is used to determine the result when the Play button is pressed. This means that the result varies 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. The sequence of numbers is repeated by most of the RNGs. Poor programming is its reason. Pseudo RNGs with very long periods are relatively easy to build. The computer that is able to complete a single period in the expected lifetime of the universe hasn't been invented yet. It was Ronald Dale Harris, a former slot machine programmer, who knew 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. However, this is impossible for most machines, because the RNG picks numbers even when the machine is not being played. So the numbers can't be exactly found in a 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. The minimum theoretical payout percentage varies among jurisdictions. 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 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 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. Only Gaming Control Board officials can change it. Other jurisdictions randomly audit slot machines to ensure that they 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
Often machines are linked together in a way that allows a group of machines to offer a particularly large prize, or jackpot. 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.
Sometimes multiple machines form multiple casinos. The machines are then owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss
The reel display of modern slot machines is controlled by computer software. Combinations that are close to winning combinations can be displayed on the slot machine intentionally.
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. This means it is much more likely that a winning combination will appear above or below a pay line than on 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.
There was an investigation held by the Nevada Gaming Commission concerning the issue of a near-miss above or below the pay line. 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. 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. 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. They are designed with advanced anti-cheating and anti-counterfeiting measures and are difficult to defraud. An example of modern ways of slot machine defrauding is directing microwaves toward it to disrupt its proper functioning.

I tis important to now what you are playing.
Only the correct amount of money should be inserted into a machine. Before leaving a machine cash out your credits.
While playing just have a good time. Use money management. Increase your bets when you are winning, and decrease your bets when you are losing.