Not since pre-history, but at least from classical times. I've come across comments the ancient Egyptians knew the shape of the Earth, but I have yet to find any substantiation for it. Pythagoras may have determined the shape of the Earth from the shadow it cast on the Moon during a lunar eclipse (which would put knowledge the Earth was a sphere as early as the fifth or sixth century B.C.).
The fact that the shape of the earth is evident by the shape of the shadow during a lunar eclipse, if not the shape of moon and sun themselves indicates to me that the spherical nature of the earth was known to prehistory. Another common observation that would suggest non-flatness is the disappearance of boats over the horizon -- since the water level is flat.