I'm not really sure how you're doing this, but I tested out a command to return the f: value in the F3 menu (Tells if you're facing north/south/east/west) I haven't done much with blocks, so I'm not really sure if its possible to get the username or EntityPlayer of the person who placed it.
//ICommandSender is used in commands... Which is what I tested this out on.
//EntityPlayer.rotationYaw is what contains the direction
EntityPlayer player = MinecraftServer.getServer().getConfigurationManager().getPlayerForUsername(icommandsender.getCommandSenderName());
icommandsender.sendChatToPlayer(ChatMessageComponent.func_111066_d(String.valueOf(player.rotationYaw)));
And that should send a message of a value between -359 and +359... in order to make sure you get the right value just do something like
float yaw = player.rotationYaw;
if(yaw < 0)
yaw += 360;
Anyways, the values would be
0 = South
90 = West
180 = North
270 = East
And you should be able to figure out the half-directions from there.