How do you know that people millions of years ago ate meat?

Scientists look at the teeth of fossils of our earlier ancestors and can see that they were omnivorous, just like modern humans. They have also found evidence of fossilised bones that were scraped with bone knives to remove the meat, something that only people could have done.

Life on Earth goes back some 500 million of the 4-5 thousand-million years of the planet's own existence as far as can be ascertained, but humans have been around for less than one million, though our nearer primate ancestors go back perhaps 4 million. So the ages of the scraped bones would be tens or hundreds of thousands of years.