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. It is a Random Number Generator that chooses the reels positions. There is a Random Number Generator in the machine's software.
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. The result varies depending on exactly when the game is played. The result is different every fraction of a second.
The numbers generated by the RNG seem to be not exactly random. Most RNGs will repeat their number sequence. Poor programming is its reason. To build pseudo RNGs with very long periods is relatively easy. 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, had access to the pseudo RNG code and seed values. 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, 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
Slot machines are typically programmed to 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. A certain winning pattern exists 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 players get the rest of the money.
When a slot machine's software is written its theoretical payout percentage is also set. To change the payout percentage after a slot machine has been placed on the gaming floor a physical swap of the software is required. 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. Gaming Control Board officials should be called for changing 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. The change cannot be done instantaneously, but only after the selected machine has been idle for at least four minutes. 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 Linking
Offering of a particularly large prize, or jackpot becomes possible when a group of machines is linked in a special way. 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. A single slot machine's jackpot is considerably less than the progressive jackpot.
Multiple machines can be sometimes linked across 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 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.
This term is also used for a related phenomenon. 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. This occurs if abbreviated physical reels are used to display a win pattern based upon the RNG. Video slot machines have virtual reels and 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. 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. The machine can't show winning combinations 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.

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. 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.

I tis important to now what you are playing.
You should know exactly how many coins you are supposed to insert into a machine. Cash your credits and only the leave the machine.
Only if you have fun your gambling won't be stressful. You can increase your bets only when you are winning, and decrease your bets when you are losing.