From Second Life Wiki
Function: key llGetLandOwnerAt( vector pos );
Returns a key that is the land owner at pos.
If the land is in public domain NULL_KEY is returned. This land can be claimed by anyone with available tier. Land on the Preview grid is divvied out this way. Land on the main grid never enters public domain and is instead auctioned off by LL.
If the land is group-owned, the group key is returned (but this isn't usable for anything, since group keys cannot be used by other functions in LSL).