Sometimes you can break out pre-Optics if you're a little sneaky. Turn on Show Tile Yields (there's a button for it at the bottom right) and look for squares 2-diagonal from your coastline that DO have some yield. Any such squares that you might find indicate the presence of a new land mass within a knight's move away from that square. (They indicate that a theoretical city could get a yield from that square, and logically that theoretical city must be somewhere else, since no square on your island can reach it.) If you build a city nearby such a square, your culture can form a bridge to the new landmass!