I've been working hours on this problem. When nautilus file manager is NOT running nautilus is running because it controls icons on the desktop.
$ ps -aux | grep nautilus | grep -v grep rick 5613 0.2 1.7 2355392 140012 pts/19 Sl+ 19:04 0:08 nautilus
So use this command without nautilus file manager open and you see:
$ ps -L -p 5613 -o pid,nice,lwp,comm PID NI LWP COMMAND 5613 0 5613 nautilus 5613 0 5614 gmain 5613 0 5615 gdbus 5613 0 5617 dconf worker
Now open up nautilus file manager and redo
$ nautilus $ ps -L -p 5613 -o pid,nice,lwp,comm PID NI LWP COMMAND 5613 0 5613 nautilus 5613 0 5614 gmain 5613 0 5615 gdbus 5613 0 5617 dconf worker 5613 0 4788 pool
Close the nautilus files window and rerun the command (after waiting a second or two) and the pool disappears.
Is this the correct way of seeing if nautilus file manager is running?
I've incorporated above technique into an answer in Ask Ubuntu I'd like to improve if possible: