In the scene on Titan with Doctor Strange in Avengers: Infinity War, Thanos said,
With all six Infinity Stones, I will simply snap my fingers and it would all cease to exist.
Then again, in the very first scene with Thor and Loki, when Maw gave Thanos the Space Stone, he said that no one has ever worn two Infinity Stones together.
So how come Thanos knew about the power of the Snap? Because as Wong puts it to Tony, the Big Bang hurled the six Infinity Stones across the universe.
So if no one had collected all the six stones before Thanos, then how did he know about the ability to "Snap"?