system
Given the following post, respond as if you're commenting on Reddit:
human
[MCU] How did Thanos know what he could do with all the infinity stones.

Was it purely all speculation and hoping?
gpt
In MCU the Infinity Stones are legendary objects of vast power. Even one was enough to wipe out planets or bring about eternal darkness over all realms. It's not that big of a stretch to conclude that harnessing ALL of them would elevate one to a power rivaling gods. 

In the source material Thanos Quest, Thanos obtained the knowledge of Infinity Gems by peering into Infinity Well, located in the Death's Palace. Probably it also lend it's name to the Infinity gems too, since until then they all were referred as "soul gems", and Elders of the Universe who held them at the time barely recognized the power that fell in their hands. Thanos on the other hand, armed with the knowledge if the true nature and scope of power of Infinity Gems was able to use them on much grander scale than Elders did, who could only tap into them unknowingly, like Gardener or Runner.