So one easy way to disprove this Flat Earth theory is the simple and humble yagi television antenna (that weird spiky metal antenna on the roof of your house that is able to receive television signals from a distance).
If the Earth was flat, you should be able to receive almost virtually every television channel in the world (you may be able to have weakened signals from mountains and other things obstructing said signals but it shouldn't matter that much). The reason for this is because television signals only travel in one direction and they get obstructed by any sold Earth features, you will not be able to receive broadcast signals underground and the curvature of the Earth blocks signals coming from other countries to reach to your part of the world; television signals do not curve around the Earth as they only go in a parallel direction.
Having a any basic understanding of physics should tell you that those Flat Earth theories in general are just bullshit.