The xwininfo (X window information) command can be used to display information about a window. The xwininfo command will prompt you to select a window:

[root@server1 ~]# xwininfo
xwininfo: Please select the window about which you
          would like information by clicking the
          mouse in that window.

Clicking the mouse on the desktop will then display information about the desktop. The output will include the resolution of the desktop, which is 1366 x 768 in this example.

xwininfo: 
Windows id: 0x1e00007 "Desktop"
  Width:     1366
  Heigth:    768
  -geometry: 1366x768+0+0

The xdpyinfo (X display information) command displays information about an X server. This command produces a list of capabilities of the X server, and the current configuration of the X server. This command will produce quite a bit of STDOUT. Near the end of the STDOUT will be a handful of listings that starts with "visual." 

[root@server1 ~]# xdpyinfo
visual:
  visual id: 0x5b
  class:     TrueColor
  depth:     32 planes
  available colormap entries: 256 per subfield  red, green, blue
  masks:     0xff0000, 0xff00, 0xff  significant bits in color
  specification: 8 bits