I'm using Ubuntu 15.10 and i want to know which display server is running. In first version of Ubuntu it is definitely X server, but recently i got updated my Ubuntu 15.10 and i came to know that some Mir Server packages are going to be installed so how will i know that either Mir is running or X Server?
(Trying to improve @shcherbak answer.)
To get info about the running display server write in the console:
ps -e | grep tty
The out put might be, for example:
1475 tty2 00:00:00 gdm-x-session 1478 tty2 00:00:40 Xorg 1489 tty2 00:00:00 gnome-session-b
For example, the results show
1478 tty2 00:00:40 Xorg, so this would be running Xorg (that is, X11).
Shscherbak answer would just output the running processes that contain a capital X, "X". Among others, of course, Xorg, in case it is present. But if another display server is running, you miss the information. :)