Good grief. There was no internet of course, and no gaming magazines in my home town.
People read instructions on the machine, and also, tried different things.
Everyone I ever met ever, learned such gaming combos, with their friends, by trial and error.
Yes, that is how it was done.
We really were guessing different combos, in rural Iowa at least.
We grew up in alternate realities then. No one just guesses MK2 fatalities. Especially not in the 3 sec time frame you have to pull it off.
To be fair, fatalities are not generally discoverable by trial and error, unlike combos. You have such a small window of time to enter them when they show up at the end of the match. The first time I encountered one, I had no idea what was going on and then a few seconds later it was over.
However, back then we had gaming magazines and books that revealed all the fatalities, so you could easily look them up.