From my Desktop I want to connect to the Raspberry:

me@mydesktop$ ssh -v -Y pi@pi
password:

pi@raspberrypi ~ $ xauth list
pi@raspberrypi ~ $ echo $DISPLAY
localhost:10.0

pi@raspberrypi ~ $ xkill
debug1: client_input_channel_open: ctype x11 rchan 2 win 65536 max 16384
debug1: client_request_x11: request from 127.0.0.1 51558
debug1: channel 1: new [x11]
debug1: confirm x11
X11 connection rejected because of wrong authentication.
debug1: channel 1: free: x11, nchannels 2
xkill:  unable to open display "localhost:10.0"

pi@raspberrypi ~ $ xhost +
-bash: xhost: command not found