It is possible to tell that the earth is round without using technology. You just have to look at the shadow of the earth on the moon during a lunar eclipse.
During the partial phases of a lunar eclipse, the shadow of Earth can be seen on the surface of the Moon. It casts a circular shape. This proves that the Earth must be round. If the Earth wasn't round, then lunar eclipses wouldn't be possible. Even if they did occur, everyone on Earth should be able to view them, but this isn't the case. Greek philosopher Aristotle found that during lunar eclipses, the Earth's shadow on the face of the sun is curved. This curved shape presents itself during all lunar eclipses, despite the fact that the Earth is rotating. This means that the Earth is curved all around; that is, a sphere. Solar eclipses also support the idea that plants, moons, and stars are all spherical objects. If the Earth was flat, and all other space objects floated above the Earth, total solar eclipses would not be possible. Even if a solar eclipse did occur over a flat Earth, everyone on Earth would theoretically be able to see the solar eclipse. This is not the case. When a solar eclipse happens, only certain parts of the world are able to view the phenomenon because the Earth is spherical.
The reason the shadow on the moon is round could be because the Earth is a flat disc. The disc would then project a circular shadow onto the moon. Eclipses are possible with a flat Earth, and eclipses prove that the Earth is flat. If the Earth was actually a spherical object orbiting through the solar system and rotating from west to east, then the moon should move from east to west just as the sun does. However, the moon does not do that. The Earth is flat because if it were spherical, the shadow that the moon casts on Earth would have to be much bigger than the moon itself. 
[P1] If the Earth is round, then there would be a round shadow on the moon during a lunar eclipse. [P2] Solar eclipses would not be possible if the Earth wasn't three-dimensional.
Rejecting the premises
[Rejecting P1] A flat Earth can also reflect a round shadow on the moon during a lunar eclipse. [Rejecting P2] Solar eclipses are possible with a flat Earth.