And you know this because?
Because our space age civilization has fashioned innumerable items that we know should retain their basic form (sufficiently enough to establish their origin) longer than 2-3 billion years. Moreover, I can deduce this from reviewing what has in fact survived from 2-3 billion years ago, and comparing the resilience of that to the resilience of items that would evidence our civilization is not a difficult exercise.
PS. Unless a meteor strikes it, the flag on the moon will be enough.
Somewhere in the depths of my magazine piles, I actually have a quasi sci-fi description of what should still be around to evidence the passing of our civilization depending on how many eons had passed. If I remember it correctly (it's been over a decade since I read it) I believe that the final item that basically goes when the sun in its death throes burns up the earth was the remnants of the Great Pyramid at Giza. Naturally that is just chosen for effect, but the basic point is that something would still be around - even then.