How to check what display-manager your using?

The default answer is to do:
$cat /etc/X11/default-display-manager
but that doesn't work on Gentoo.
Also

echo $DESKTOP_SESSION

returns "default" which obviously is not very useful.

So what worked for me is either:
$cat /etc/systemd/system/display-manager.service
or
$systemctl status display-manager
In my case, it's lightdm and  lightdm.service.