How to find out xorg version? or "what's my xorg version?"

You can take a look at the version reported in your packaging system, in Debian:

$ apt-cache show xserver-xorg | grep Version
Version: 1:7.5+8

In this example the version should be 7.5.

On Ubuntu 16.04, I used:

sudo X -version

It is a bit confusing, because the official documentation refers to the latest versions as X11R7.6 and XServer 1.9. Based on these numbers, you are running "X.Org 7.6" as your X.Org Server is at version 1.11.1.902.