You know TCP is disabled if you do
netstat -na|grep 6000
and you see nothing. If X11 would be listening you should see something like:
tcp4 0 0 *.6000 *.* LISTEN
tcp6 0 0 *.6000 *.* LISTEN
Also when you try to start an X11 app (like xcalc) from the terminal and you have set the DISPLAY variable to localhost:0, then you will see this if the X server is not listening on TCP (after a few seconds):
Error: Can't open display: localhost:0
To enable TCP on the Xserver type this into a terminal:
defaults write org.x.X11 nolisten_tcp 0
and then restart you X server. Now xcalc should start immediately.