I'm not exactly sure what you want to do.
From what I understood you want a function that checks if there are certain blocks in certain locations, and return if there isn't?
If so your method should be a boolean rather than a void, and then return false if any block isn't what you want it to be, and then otherwise return true.
Something like
public boolean blockCheck(World world, int x, int y, int z)
{
if(world.getBlock(x,y,z) != Blocks.grass
|| world.getBlock(x + 15, y, z) != Blocks.grass
|| world.getBlock(x + 15, y, z + 12) != Blocks.grass
|| world.getBlock(x, y, z + 12) != Blocks.grass)
{
return false;
}
return true;
}