It's really hard, actually! If you recognize that, that's a plus. Typos are rare by the time material goes to a proofer. The mistakes I find are usually in places where the eyes just glide over, not in the places where you think they'll be, and there might not be a real error per se at all -- just a client style preference.

If there's a date -- say, Tuesday, March 6, 2024 -- you have to check it. That date is wrong because March 6 is a Wednesday, but that's the sort of thing you have to check. And you can check 50 things but still miss some odd little thing that didn't strike you at all. It feels like a gut-punch to miss something.