How can I find which desktop enviroment I am using?

And you can try $DESKTOP_SESSION for the DE.

This command seems to be useful:

ls /usr/bin/*session

• in GNOME returns /usr/bin/gnome-session (and more)
• in MATE returns /usr/bin/mate-session (and more)
• in LXDE returns /usr/bin/lxsession (and more)
• in JWM returns /usr/bin/icewm-session (should be jwm-session, not?!)

I've tested the best tools I've found with:

a - GNOME under Linux Mint installed
b - GNOME under Linux Mint Live USB
c - MATE under Linux Mint
d - LXDE under Lubuntu
e - JWM under Puppy Linux (JWM is not a desktop environment but a stacking window manager).
f - XFCE under Debian Buster

Results

• env | grep DESKTOP_SESSION= (so-so)

  a & b - DESKTOP_SESSION=gnome
  c - DESKTOP_SESSION=default.desktop
  d - DESKTOP_SESSION=Lubuntu
  e - Nothing
  f - DESKTOP_SESSION=xfce
  

• echo $GDMSESSION (so-so)

  a & b - gnome
  c - Nothing
  d - Lubuntu
  e - Nothing
  f - xfce
  

• pgrep -l "gnome|kde|mate|cinnamon|lxde|xfce|jwm" or ps -A | egrep -i "gnome|kde|mate|cinnamon|lxde|xfce|jwm" (well, but not perfect)

  a & b - OK
  c - OK
  d - WRONG
  e - Nothing & OK
  f - OK
  

• HardInfo (very well, but not perfect)

  a - Nothing
  b - GNOME 2.32.0
  c - MATE
  d - LXDE (Lubuntu)
  e - Unknown (Window Manager: JWM)
  f - XFCE
  

Conclusion

A combination of HardInfo and the command ps -A | egrep -i "gnome|kde|mate|cinnamon|lxde|xfce|jwm" probably will give the desired answer.