Well, since the force of gravity is roughly the same everywhere on the globe, it could be surmised that the Earth must be spherical. If the Earth were not round, whole hemispheres would have different atmospheric pressure and significantly different sea levels.
If that isnt enough for you,
http://imgur.com/C09AVIf that isn't enough for you,
then try simultaneously measuring the length of the shadows cast by identical poles perpendicular to a flat surface that is tangential to the earth's radius at various, distant locations. If indeed the earth is round, then the shadows should all vary in length from one distant location to another, which means that the angle at which the parallel rays of sunlight struck each pole varied from one location to another.
If that isn't enough for you,
you can try observe, with binoculars, ships slowly 'sinking' below the horizon as it sails farther and farther out to the ocean, then watch them come back. They certainly didn't fall off the edge of the earth!
If that isn't enough for you,
Why is the sun shineing ind china, morning in Hawaii and night in mid USA?
If that isn't enough for you,
Look at all the other planets. It'd be logical to assume since every single planet is round earth would be also. Please dont say earth is "SPECUL" because it has life and others don't.
This along with a lot of math that would be in accurate, say targeting using the formula to measure the earths surface.
<3 Connor.