How as an individual can I know if the Earth is a sphere or a flat disc? What experiment can I do that doesn't involve trusting information from a 3rd party that would prove what the geometry really is?
If it is flat then what is in the opposite side of it?
In order to believe the earth is flat you need a serious lack of knowledge in physics and critical thinking.
Just a simple 3D model of what they think the universe is like shows how it's just not possible;
https://www.youtube.com/watch?v=3_5Fz4Qa6_8
If you want to do an experiment without third party;
1. Take a long stick, put it plumb on the ground, look at the time and measure the length of the shadow.
2. Travel a proper distance east or west and do the same at the exact same time.
3. Calculate the what it should be according to your model and according to earth-being-a-globe model.
4. If the calculations are too difficult I'd be glad to help
This was done 2500 years ago as far as we know to figure out earth's radius
Why would anyone do the effort of making everyone believe the earth is flat? So you think all of science is just one big hoax?
I find all of this disturbing, I think people who believe the earth is flat with current availability of information suffer from psychosis and have a severe lack of understanding everyday things, quite worrying.