The RNG
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. It is included into the machine's software.
The RNG generates random numbers at an extremely high rate. When a player presses the Play button the most recent random number determines the result. The result is different depending on exactly when the game is played. The result changes every fraction of a second.
Some professional gamblers observe that the RNG does not actually generate random numbers. Most RNGs will repeat their number sequence. The cause of it is in poor programming. It is relatively easy to build pseudo RNGs with very long periods. There is no computer that is able to complete a single period 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. He 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.

Payout Percentage
Winnings 82–98 per cent of the money that is wagered by players are typically paid out by slot machines. This payout percentage is called theoretical. The minimum theoretical payout percentage varies among jurisdictions. Law or regulation typically establish it. 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. As for the rest of the money it goes 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. Being a time-consuming process it is done not very often. Certain jurisdictions presuppose the EPROM to have a tamper-evident seal. 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 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.

Linked Machines
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.
There can also be multiple casinos consisting of multiple machines. In these cases, the machines may be owned by the manufacturer, who is responsible for paying the jackpot.

Near-miss
Computer software controls the reel display of modern slot machines. Combinations that are close to winning combinations can be displayed on the slot machine intentionally.
The term near-miss programming is used to denote this practice.
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 phenomenon only occurs where 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 the subject of the Nevada Gaming Commission investigation. They ruled that this was legal, so long as the near-miss above or below the pay line was not specially programmed. That means that the possibility of near-miss occurrence should be equal to the possibility of 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. Stop motion cameras are used by regulators for manufacturer's practices audit.

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. These machines and their bill acceptors 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.

To know what you are playing is necessary for you.
Always make sure that you insert the correct amount of coins into a machine. Before leaving a machine cash out your credits.
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.