#1 Pick em! When you get a bonus and you have to pick to reveal prizes or other features, these are referred to as picking bonuses. Slot picking bonuses are not predetermined, well, at least most of the time they are not. There are a couple manufacturers that have predetermined picking bonuses. Have you ever played Aristocrat's Outback Jack? Outback Jack is a perfect example of a predetermined picking bonus. The easy way to tell if a picking bonus is predetermined or not is to observe one thing: If the slot reveals the unpicked items when the bonus is over, it is not predetermined. No show = predetermined. Love em or hate them, it's generally your fault if the bonus ends early or the prize is not as big as you had hoped.
#2 Free Spins: Free spin bonuses are a different animal but are equally random, one could say very random. Each free spin within the bonus is chosen by separate Random Number Generator (RNG) pulls - one for each reel to be precise. Without overcomplicating this, just know that the outcome of each free spin in the bonus is determined when you either press the play-on (bet) button or when the credits won (if any) are done counting up. For losing spins, the RNG will select the results for the next free spin once the losing spin is complete. Any subsequent retriggers and wins (or no wins) are chosen at the time the individual spin begins...not when the bonus begins.
#3 It's been forever! The frequency of a slot bonus varies wildly from an average of once every ten base game spins up to as high as once in 150 spins. In some cases, certain bonus features will trigger even less often than that! An estimated industry average on a medium to high volatility machine is once every 80 spins. In general, the frequency of a slot bonus is a good way to measure the volatility of any given slot: More frequent = less volatile. Keep in mind, many other factors go into determining the volatility of a slot - visit here for more.
#4 Be happy! Free Spin bonuses are often advantageous over the base game spins. That's why people get excited about the prospects of a bonus and why they are often filmed and uploaded to YouTube. So, what are the advantages? Well, they come in different forms, but the most common are win multipliers, additional wilds (sometimes sticky), and extra credit awards. WMS' Jungle Wild 3 uses two of those features in its bonus.
#5 Not so fast! Those increasingly popular wheel bonuses you see on the tops of machines like Tarzan and Super Monopoly Money - guess what, they are weighted. Weighting means that the smaller prizes are generally awarded more frequently and the larger prizes rarely. It may appear you have an equal chance to land that 100X or that Grand Progressive, but in reality those lofty awards are nothing but a tease. Don't chase them and don't feel bad that you missed a big one by a hair - it was never meant to be.