You'll always have to check if the tile fits when put side by side.
Like Sol said, it's like a jigsaw puzzle, but with only one piece that should fit in every side.
Propably the easiest but definetely not the best way to make the tiling look natural, is taking one half of the tile, copying it and then mirroring it to the other side of the sprite.
I'd suggest you just try and see that the left side of the tile fits with the right side, same goes with up and down sides.