A lot depends on the definition of "know". To me i cannot "know" anything 100% as it could be that i am just a figment of imagination in some being's mind.
So, to me "know" is to be 99.99% sure and this does not require the 100% sure one might get if chess is ever solved.
If you play chess at a high level for decades--you learn a lot about chess and you "know" a lot about chess.

Another piece of evidence that perfect games have been played is that out of billions of games played--noone can show even 1 game played where someone won without his opponent making a mistake. So the game of chess is rather obviously a draw.
and tens of thousands of games have been analyzed and there has been found to be a lot of leeway before a game turns away from a draw.