tl;dr You need a camera and either a long lens or a telescope that'll hook to it to see how parallax lets you view the moon from different angles. So, the moon always shows one face to Earth. Round Earthers believe it's because, despite the moon being a sphere, it's "tidally locked" to Earth and stuck in that position. Sounds suspicious, eh? Well, Round Earthers also believe we are also on a big sphere, and that sphere is constantly turning. If that's true, we're also shifting in position in relation to the moon. If the moon was a sphere you'd be looking at it from different angles, wouldn't you? Well, turns out you can. If you can take good quality photos of the moon at dawn and dusk you'll see the moon from different angles. If you take a number of different photos over night and string them together you'll be able to see the angular change. With a good view (you'll need a 1000mm equivalent lens or better) the 3D motion is apparent enough to demonstrate both that the moon is a sphere and that you're moving in relation to the moon.