We can be sure that the earth is flat because of four reasons.
1. Look out your window, do you see a sphere? You're talking out your ass if you think there's a flat surface on a sphere, there's not. Go pick up an apple, let me know when you find one with a flat surface on.
2. The conspiracy. It exists, the number of NASA whistleblowers (jfgi) confirms that. They have to be hiding something.
3. round earther science is bs. Once the earth was a sphere, now it's some oblate whatever, then it's tilted on its axis. You keep changing stuff and making it up try to escape the unavoidable truth. Ancient people knew the earth was flat, let's stick with the view that hasn't changed. shall we?
4. even round earth science confirms a flat earth. Everything in the universe is accelerating away from the site of the big bang. Universal acceleration, bingo. if universal acceleration exists, matter will be pushed along on it. There's no opportunity for it to push back and form a sphere, it ends up a disc. Try it sometime. Pour sugar out onto your worktop, push it along, let me know when it ends up a sphere or oblate whatever.
The earth is flat, deal with it.