When you tell flat-Earthers that the Earth is not, in fact, flat, one common—and insipid—comeback is: "How do you know, have you ever been to space?" The conceit behind this question is that you cannot know anything unless you know it first-hand. This ridiculous statement has at least two problems. The first is that it's simply not true. The second is that it represents the height of hypocrisy coming from people who believe the Earth is flat, for although they will claim to "know" this on the evidence of their own eyes, nothing could be further from the truth.
How much do you know first hand? What do you do, on a daily basis, to personally verify the information that you put to practical use? Do you go around sticking your finger in live light sockets to verify that electricity might shock you? Do you put liquids other than gasoline in your tank to see if they are better for running your car? Do you personally verify every thing a candidate has ever said before you vote for him or her?
Of course not. And as the technology we use gets more complex and connected, we know less and less about it first-hand. But we have good reason to believe that at least most of what we are told is true, especially if we know that there are enough people out there with access to the truth who could, at any time, verify or falsify the information.
In science, even among professional scientists, it is wholly impractical to know everything first-hand. Every experiment would have to be repeated every time. True, any new result does need to be verified, several times, until it is established that the results are valid (or that they are not, which happens quite often). But once results are established, then they can be built upon to advance knowledge about the world, and the Universe.
But flat-Earthers don't even do this much. They will sing the praises of first-hand knowledge on the one hand, and then on the other hand post endless memes from others with no attempt to verify any of the information, if it can be called that, that the memes contain. They will tell you that they know that the Earth is surrounded by an ice wall.
"How do you know? Have you been there?"
"Of course not, they won't let you anywhere near it."
"How do you know that?"
"Have you read the Antarctic Treaty?"
"Yes, have you?"
"UN Troops guards the ice wall. They will shoot you."
And on it goes. The most infuriating thing about this is that, unlike many things we have to trust other people to be honest with us about—whether a certain medicine is right for what ails us, or if that noise in our engine is anything to worry about, for example—whether or not the Earth is flat is something anyone with a tiny bit of math skill, or the ability to use online calculators, can indeed verify first hand, without traveling to Antarctica to see if troops are waiting to apprehend you or traveling into space.
But so far, none of the flat-Earthers have taken me up on the challenge.