65 million years since?

All civilizations leave identifiable traces of themselves (the ruins of their cities). In a million years, our cities would still dot the landscape.

what?

Yeah, this isn't at all true.

We've stripped much of the world bare of certain natural resources, altered the physical landscape, set off nuclear weapons distributing trace elements worldwide, and left crap piles which won't decay in millions of years.

Any marginally advanced civilization will recognize our footprint.

We'll never know. When it comes to direct evidence of an industrial civilization—things like cities, factories, and roads—the geologic record doesn’t go back past what’s called the Quaternary period 2.6 million years ago. For example, the oldest large-scale stretch of ancient surface lies in the Negev Desert. It’s “just” 1.8 million years old—older surfaces are mostly visible in cross section via something like a cliff face or rock cuts. Go back much farther than the Quaternary and everything has been turned over and crushed to dust.

We now know a wide panel of Cretaceous species.

Some estimations have been done about dinosaur intelligence (dinos were the dominant group by then and the most likely to have built a "civilization").

Brain to body size ratio is a first rough approximation.

It seems theropods (raptors, T. rex...) might have been the smartest among late Cretaceous species).

Moreover, birds, which are theropods too, have a quite efficient brain: they can be smart despite a relatively light brain (parrots, corvids...). So raptors might have been smarter than modern days mammals with a similar brain.

Another evidence of intelligence, are those bigger dinos, dead from multiple bites of smaller raptor jaws. This suggests a planned attack by a pack of organized small predators.

According to various authors, smartest raptor intelligence may vary between opossums and wolves.

This is amazing but certainly not enough to build what we call a "civilization".

Dinosaurs weren't a civilization. We could easily find evidence of such a thing. The composition of the earth would be different.

Well, those are the same questions that everybody else has been asking for years. If you look, I'm sure you could find some scholarly articles on the subject.

easy, how do we know dinos existed?

No one can say for sure that there haven't been other civilizations before us. Archaeologists and paleontologists just haven't YET dug up anything older than the city at Gobekli Tepe in southern Turkey, which dates back about 12,000 years. But they might find something older in the future!