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. It is a Random Number Generator that chooses the reels positions. The machine's software includes a Random Number Generator.
The RNG generates 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. 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.
The RNG was noticed to generate not exactly random numbers. Indeed, most RNGs will eventually repeat their number sequence. This behavior is due to poor programming. To build pseudo RNGs with very long periods is relatively easy. 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. Specific gambling games equations were discovered by him. The fact that the next set of selected numbers would be based on the previous games played was revealed. 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.

Percentage Of The Payout
Winnings 82–98 per cent of the money that is wagered by players are typically paid out by slot machines. 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. A certain winning pattern exists nearly in every casino. This is the amounts they pay and the frequencies of these pay-outs. 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 goes to the players.
A slot machine's theoretical payout percentage is set at the factory when the software is written. To change the payout percentage after a slot machine has been placed on the gaming floor a physical swap of the software is required. Being a time-consuming process it is done not very often. There is sometimes a tamper-evident seal on the EPROM. Gaming Control Board officials should be called for changing it. The audit of slot machines is made by some jurisdictions make sure that they 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 making the change the machine is locked to new players for four minutes, it displays an on-screen message informing potential players that a change is being made.

Machines That Are Linked
A group of machines offers a particularly large prize, or jackpot if they are linked together 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. The amount of the progressive jackpot is far higher than any single slot machine's.
Multiple machines can be sometimes linked across multiple casinos. 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. Combinations that are close to winning combinations can be displayed on the slot machine intentionally.
This practice is called near-miss programming.
This term is also used for a related phenomenon. It is winning percentages that 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. 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 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 investigated by the Nevada Gaming Commission. 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 can't show winning combinations more frequently than other combinations above or below the pay line.
Near-miss programming, where a near miss is inaccurately displayed is not allowed in Australia either. Due to this, regulators use stop motion cameras to audit manufacturer's practices.

Fraud
Mechanical slot machines and their coin acceptors were sometimes susceptible 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. Recent attempts at defrauding slot machines involve manipulating the EPROM, such as by directing microwaves toward it to disrupt its proper functioning.