Xfce power management has a Presentation Mode to disable screensaver or suspend settings
Left click Power Manager plugin in system tray to turn it on
Power Manager Plugin Properties has an option to show an additional indicator when Presentation Mode is on
xfce:xfce4-power-manager:panel-plugin [Xfce Docs]
Or use this command to toggle Presentation Mode on/off
xfconf-query -c xfce4-power-manager -p /xfce4-power-manager/presentation-mode -T