If you are wondering if the Earth is a sphere or a flat plane one good observation is the sunset.
On a flat plane the Sun would become smaller and smaller as it moves towards the horizon, like when looking at a car driving away will become smaller and smaller.
On a sphere the Sun will vanish bottom up and remain more or less the same size, there might be a small amount of distortion due to refraction.
Below you can see some observations I did with two different cameras, and on one of the cameras I from time to time used a solar filter.