Hmmm... Technically I would say that you could see the curvature by just standing on the ground. A property of a ball is that it looks like a circle from every perspective, as long as you are not inside of it. Take a ball in your hand, and you see the curvature in the sence that you see a circle. If you would look from the perspective of the "north pole" of the ball, you will never be able to see the "equator" of the ball, no matter how far away you are. But the closer your eye is to that ball, the further the distance is away from that equator. So technically, you can never see more than a hemisphere of a ball.

But no matter how close you are to that ball, you will always see a circle, and that circle IS the curvature of the ball. If you'd make a camera the size of an ant, and you place it on the ball, you'd see the "horizon" of the ball. And that horizon surounds you as a circle. That circle IS the curvature of the ball.

The same applies to the earth. The horizon IS the curvature. The horizon is that circle. And no matter how high you go up, you can never so more than a hemisphere at the same time. The fact that you cannot see beyond the horizon, is evidence for a spherical earth. It's as easy as that...