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"?

It seems to be general knowledge that with the all the Infinity Stones the wielder (if he is strong enough) can do...

Anything.

Certainly the Asgardians seemed to be aware of it...even Odin had a fake Gauntlet made up.

In addition, both Wong and Dr Strange are both aware of the Infinity Stones and their inherent nature and the power of individual stones is well documented across the previous MCU movies.

Wong: [Using magic to show the universe and five out of six Infinity Stones] From the dawn of the universe, there was nothing. Then, boom! The Big Bang sent six elemental crystals, hurtling across the virgin universe. These Infinity Stones each control an essential aspect of existence.

...

Tony Stark: [Speaking to himself] This is it....What's our timeline?

Bruce Banner: No telling. He has the Power and Space Stones, that already makes him the strongest creature in the whole universe. If he gets his hands, on all six Stones, Tony...

Doctor Strange: He can destroy life on a scale hitherto undreamt of.

^{Wikia Transcript}