The same way you know any scientific law is always true whether you're consciously looking at it or not. The same way you know that tree falling in the forest still makes a sound even when there's nobody around. In other words, you don't know, but all of science and rational existence depends on making the assumption, and there's nothing special about this case to make us stop.
i have to say that ur comment here is retardedly stupid. We know a tree that falls without anyone there to hear it makes a sound because everything that moves within the atmosphere makes the molecules of gas move and vibrate, sound is defined as vibration of atoms and molecules, therefore we absolutely DO know that a tree makes a sound when it falls and there is no one around to hear it. And for the rest of your comment...we dont JUST believe its true cuz thats what life relies on, were that true it would make scientists no better than the catholic church that supressed science cuz it was heresy. We have MATH to PROVE that its true. It may not have been observed, but its certainly not blind faith, scientists reject anything thats just blind faith. To believe its only true because we HAVE to is pure stupidity.