Another way to do it is the xlsclients command.

Also, I can reliably tell based on the fact that the contents of the XWayland windows are fuzzier.

I can do that because I have had lots of practice discerning fuzziness, because I am using a old monitor (as opposed to a HiDPI display) and because I have a fractional (non-integral) scaling factor declared in my monitors.xml file--when the scaling factor is set to 1.0 there is no difference in fuzziness as far as I can tell.

how does xeyes help with that? does it fail to track the mouse on wayland apps? i can see that would be useful.

i actually have problems with XWayland, it frequently (every few days), goes to a 100% CPU and locks up the screen. then i have to remote login and kill it, at which point i see which apps die, so i know what is using XWayland from that. :-)