The spawn chunks usually consist of an area of 16×16 chunks centered as close as possible to the world spawn point. Entities are only active if all chunks in an area of 5×5 chunks around them are loaded, limiting their activities to an area of 12×12 chunks. The exact rule includes chunks whose center is less than or equal to 128 blocks away from the world spawn along both axes. In the rare case where the world spawn is located at the exact center of a chunk, 17 chunks will be loaded along that axis, of which 13 activate entities.
There are several—sometimes elaborate—ways to determine the world spawn point:
A compass will point to the spawn point.
The following will only work for players not connected to a bed:
In Adventure mode, the northwest corner of the block players spawn on is the world spawn point.
In Creative, Spectator, and Survival mode, players spawn on a random block in a 20×20 box centered on the spawn point. The spawn point can be inferred by repeatedly respawning and keeping track of the smallest rectangle encompassing all blocks players have spawned on so far. If the rectangle spans between a < b on the x-axis, the spawn point can be on any integer x between b − 10 ≤ x ≤ a + 10 with equal probability. Likewise for the z-axis. Once the rectangle has grown to 20×20 blocks, the spawn point is its center exactly.
Entities other than players falling into the exit portal in the End will land on the spawn point exactly. Items thrown in will mark the spot in the Overworld. Players will spawn like they normally do, allowing this to be used to perform the above without dying.
A new spawn point can be set using /setworldspawn. If no coordinates are provided, the northwest corner of the block the player is currently standing on will become the spawn point. Note that this will not load the new spawn chunks; it merely prevents them from unloading. The chunks can be loaded by walking into range or restarting the server. They will then remain loaded until the spawn point is moved again.
A mod [1] or external program such as NBTexplorer or MCEdit can also be used to find and set the world spawn point.