Simple linux command that will tell me what the display manager is

From the bottom up:

• Xorg, XFree86 and X11 are display servers. This creates the graphical environment.
• [gkx]dm (and others) are display managers. A login manager is a synonym. This is the first X program run by the system if the system (not the user) is starting X and allows you to log on to the local system, or network systems.
• A window manager controls the placement and decoration of windows. That is, the window border and controls are the decoration. Some of these are stand alone (WindowMaker, sawfish, fvwm, etc). Some depend on an accompanying desktop environment.
• A desktop environment such as XFCE, KDE, GNOME, etc. are suites of applications designed to integrate well with each other to provide a consistent experience.

In theory (and mostly so in practice) any of those components are interchangeable. You can run kmail using GNOME with WindowMaker on Xorg.

IIRC neither gnome nor kde currently support dpi scaling on wayland, so the answer is currently none.

Update: Work has started on multi-DPI in Gnome 3, no indication yet on whether it would be usable in 3.22...