They'll say, "So what?" or "Who cares?" That's how you can tell. And you'll probably be talking about the only historical subject they care about enough to talk about: Judeo-Christian-Islam, aka the Abrahamic religions.
They care enough about it to talk about it. A lot. But not enough to learn anything about it. This is a puzzling paradox.
INFORMED PERSON: Jesus -- if he lived -- probably died no sooner than around AD 25, and Paul's earliest writings come from the early 50's, so the gap between Jesus' life and the earliest writings about him is not 70 years, as you just said. It's 25 years at most, and maybe closer to 15 years.
NEW ATHEIST: Oh... So what? Who cares?
INFORMED PERSON: Who cares?! You cared enough not one minute ago to say that the gap was 70 years or more.
NEW ATHEIST: So what? What does any of this prove? You're all hung up on numbers!
The New Atheist, who is quite likely an astrophysicist, scolds the informed person for being "hung up on numbers." Someone who's never dealt with New Atheists might think that there's no way my description could be at all realistic. Those who have dealt with them realize that it's as accurate as can be. These chumps have got a serious learning disability when it comes to the humanities. Which they tend to despise. Perhaps their eyes glaze over and their brains shut off when the topic turns to historical things. How else to account for exchanges like "None of the books of the New Testament were even written until more than 300 years after Jesus' supposed death." "No, the earliest ones were written within 15 to 25 years, and they were all written within 120 years or so of Jesus' supposed death at the very latest. More likely well within 100 years." And then suppose the best-case scenario, which is that sooner or later you're able to convince the New Atheist that you didn't just pull your numbers out of your butt -- get set for the inevitable reaction:
"So what? Who cares? What does any of this prove?"
What indeed? And you will point out to the New Atheist that it was he, after all, who had brought up the topic of the chronology of the writing of the New Testament as it related to the dates of Jesus' life. And the New Atheist will look at you as if you had three heads or had begun speaking to him in Assyrian.