xtruss (easy to use X protocol tracing program)

xtruss logs everything that passes between the X server and one or more
X client programs. In this it is similar to xmon(1), but intended to
combine xmon's basic functionality with an interface much more similar
to strace(1).

If xtruss hangs on startup or aborts with a 'sk_addrcopy'
message, make sure your hostname (hostname -f) resolves to
your primary IP address. Add an entry in /etc/hosts if necessary.
This is especially prevalent on residential LANs using DHCP.