According to optical researchers, while we can fool ourselves into thinking we see the curvature of the Earth from high mountains, this is usually wishful thinking.
You should be able to detect it from an aeroplane at a cruising height of around 10,600 metres (35,000 feet), but you need a fairly wide field of view (ie 60 degrees) and a virtually cloud-free horizon. The reality is that clouds, hills and mountains mean we rarely get to see the kind of perfectly flat horizon where the curve would be most obvious.
However, you can detect the curve of the Earth from ground level at the coast with a pair of binoculars – just look for distant ships on the horizon and you’ll see that their hulls start to disappear before their masts and other superstructure. Ancient Greek scientists, who spotted this without any optical aids, used this to conclude that the Earth was round.