If you didn’t know that the Earth is a sphere, there are three common observations you could use to convince yourself that it is.
The first common observation is the shape of the moon. First, the face of the full moon is circular, and that would lead you to believe that it is a sphere rather than a disc. When the moon eclipses the sun, the shape of the shadow is always circular, which clinches a spherical shape for the moon. By extrapolation, you could assume that the Earth is a sphere also.
Also notice that when the moon is being eclipsed by the Earth (a lunar eclipse), the part of the moon that is eclipsed is actually the shadow of the Earth. This shadow tells you that the Earth is a sphere just like the moon.
A third way to see that the Earth is a sphere is to look at how objects in the distance «disappear» as you get farther away. For example, a 100-foot-tall ship that is 15 miles away is not visible. That’s because it is blocked by the curvature of the Earth. As it approaches, it «rises.» First the tip of the mast is visible, then more and more of the ship comes into view as the ship gets closer.