how to determine which composite manager is running

I'm using Arch Linux, and have both Gnome and the i3 window manager installed. When running i3, I'm trying to initiate the composite manager Compton. But trying to do so results in the following error: $ compton [ 04/11/2019 22:32:36.443 register_cm FATAL ERROR ] Another composite manager is already running I think this means that Compton is already running, or Mutter is running, but I'm not sure which. **Is there a command I can use to determine which composite manager is currently running?**