You mean how to check or verify what it's using or has used at all for the whole time it runs under that desktop?
Other than that, I can go with port options, dependencies, inspecting
Makefile's, and
pkg info
listing of a port. Also checking the source code if there's any possible calls to specific library or other code, which would narrow down which libraries are being used, but maybe not fully identify it.
As for ports in the virtual category of
Wayland, they likely use Wayland natively, and don't rely on xWayland at all. Most are built on top of qt or gtk, so those are toolkits which run on it natively.
SDL is another contender for Wayland, so
devel/sdl12-compat could be that a for SDL, as it's the only SDL named application in the category of
Wayland. Unfortunately, that port is for SDL 1.2. Maybe, not enough investigation has been put into SDL on FreeBSD for Wayland yet. SDL is intended to run natively on Wayland. Edit: Other gaming libraries and engines were intended to use SDL as a compatibility layer, so they could be used on top of Wayland. It would be better if they develop their own layer for native Wayland use; maybe they eventually will.