Watch a total lunar eclipse is one way. The only shape that can cast a ciruclar shdow no matter which direction the light comes from is a a shore.

Go to the top floor of very tall building and look toward the horizon. The hight of your eyes ABOVE the ground determines how for away the horizon is It's about 3 miles for someones whose eye highit is 5 feet 7 inches above ground level. The curvature of the horizon is very slight from Pike's Peak, but it is there. and i was nowhere NEAR being 6 feet tall, I never have been, when i was 11 10 or 11 years old..

Take flight in plane and look at the horizon..Even a small prvate plane or even a glider you should be able to see a SLIGHT curvature of the horizon.

Watch a boat disappear over the horizon large lake or ocean.

Of course for some people, nothing will ever convince them that the Earth really is round, spherical, or slightly oblate spheroid. Even the ancient Greeks and Romans knew Earth was spherical. So did the Ancient Chinese or the not so ancient Chinese who discovered north and south America decades before the most Europeans.. So did the Norse.