Monitor DBUS signals: dbus-monitor

• Send desktop notification: notify-send test
• Send desktop notification with icon: notify-send -i face-glasses "test" (icons are located in /usr/share/icons/)
• Monitor DBUS desktop notifications: dbus-monitor "interface='org.freedesktop.Notifications'"

Primary source

Daemons:

• dunst
• NotifyOSD / notify-osd

Depending on the Desktop Environment, notification daemons are built-in (Cinnamon, Gnome, KDE). Notification daemons can started standalone for other environments (e.g. i3wm). Standalone is possible via XDG autostart or as DBUS service.

Typically, DBUS service files are shipped with notification daemons, e.g., /usr/share/dbus-1/services/org.freedesktop.Notifications.service