I have run a blender instance in the background using
Now I want one panel to get visible only if there is a blender instance running in the background.
This is no different from regular subprocess use: https://docs.python.org/3.6/library/subprocess.html
Use the return value of
Popen and check it will
See: https://stackoverflow.com/questions/12057794 - though in your case you won't want to wait until poll finished.