How Do You Find What GUI/Desktop Environment Your Linux Server Is Using?

Problem scenario
There is a GUI desktop environment that your Linux machine is using. You want to know which one it is. What do you do?

Possible Solution #1
Run these commands:
sudo ps -ef | grep gnome
env | grep -i xdg

Possible Solution #2
echo $XDG_CURRENT_DESKTOP
echo $DESKTOP_SESSION

(Taken from https://unix.stackexchange.com/questions/46305/is-there-a-way-to-retrieve-the-name-of-the-desktop-environment)

Possible Solution #3
This command may give you clues if it is gnome: whereis gnome

Possible Solution #4
In the desktop GUI, go to Systems Tools or YAST (or some Control Panel equivalent) and go to "About". It may say there.

Possible Solution #5
apt list --installed | grep gnome
sudo yum list installed | grep gnome
sudo zypper se | grep gnome

If you suspect something other than Gnome is installed, see this page for other choices: https://en.wikipedia.org/wiki/Desktop_environment