The RNG
Nowadays slot machines are computerized. So the odds are whatever they are programmed to be. In modern slot machines, the reels and lever are present for historical and entertainment reasons only. The positions of the reels depend on a Random Number Generator. The machine's software includes a Random Number Generator.
The RNG is constantly generating random numbers at an extremely high rate. When a player presses the Play button the most recent random number determines the result. This means that the result varies depending on exactly when the game is played. The result would be different a fraction of a second earlier or later.
It was noticed that the RNG does not actually generate random numbers. It turned out that The number sequence is repeated time after time. This behavior is due to poor programming. Pseudo RNGs with very long periods are relatively easy to build. 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. Specific gambling games equations were discovered by him. 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 player cannot see the secquence.

Payout Percentage
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. The minimum theoretical payout percentage varies among jurisdictions. It is typically established by law or regulation. Nearly each casino has its own winning pattern. 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 players get the rest of the money.
At the factory when the software is written a slot machine's theoretical payout percentage is set. 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. One can find a tamper-evident seal on the EPROM in certain jurisdictions. To change it Gaming Control Board officials should be present. 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. The change can be done only after the selected machine has been idle for at least four minutes. 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
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. A single slot machine's jackpot is considerably less than the progressive jackpot.
Multiple machines can be sometimes linked across 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. Combinations that are close to winning combinations can be displayed on the slot machine intentionally.
Such showing combinations that are similar to winning combinations more frequently than would occur randomly is called near-miss programming.
There is a related phenomenon which is also 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. This occurs if abbreviated physical reels are used to display a win pattern based upon the RNG. The symbols that appear around the winning line in video slots are 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. In other words, the near-miss must be just as likely to occur as any other combination. The machine cannot be specially programmed to 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. Manufacturer's practices are audited by regulators with the help of stop motion cameras.

Slots Fraud
Mechanical slot machines and their coin acceptors were sometimes susceptible to cheating devices and other scams.
Modern slot machines are controlled by EPROM computer chips and coin acceptors have become obsolete in favor of bill acceptors. They are designed with advanced anti-cheating and anti-counterfeiting measures and are difficult to defraud. Nowadays microwaves are used to defraud slot machines.

I tis important to now what you are playing.
Always make sure that you insert the correct amount of coins into a machine. Your credits should be cashed out before you leave the machine.
Just have fun. Slots should be fun, not stressful. Be smart and change oyur bets according to your luck.