A simple way, you can take a look
echo "$XDG_CURRENT_DESKTOP"
in the terminal.
Let's do it faster, to check what is running inside the seat.
Here is a one-liner, just run it in your terminal:
desktop="${XDG_CURRENT_DESKTOP:-${XDG_SESSION_DESKTOP:-}}"; session_type="${XDG_SESSION_TYPE:-}"; info="" session=""; if command -v loginctl >/dev/null 2>&1; then session="${XDG_SESSION_ID:-self}"; info=$(loginctl show-session "$session" -p Desktop -p Type 2>/dev/null) || info=""; if [[ -z "$desktop" ]]; then desktop=$(sed -n 's/^Desktop=//p' <<< "$info"); fi; if [[ -z "$session_type" ]]; then session_type=$(sed -n 's/^Type=//p' <<< "$info"); fi; fi; printf 'DE=%s\n' "${desktop:-unknown}"; printf 'TYPE=%s\n' "${session_type:-unknown}"
It will show something like
DE=KDE
TYPE=wayland