ask | dr. root

Two options...

In a Terminal window, you can either type:

# xdpyinfo

'xpdyinfo' is a utility for displaying information about an X server, including the types of screens available, their resolution and color depth.

screen #0:
dimensions: 1280x1024 pixels (361x289 millimeters)
resolution: 90x90 dots per inch
depths (3): 1, 24, 32

Or use the 'xwininfo' utility with the added '-root' option:

# xwininfo -root

xwininfo: Window id: 0x60000f

Absolute upper-left X: 0
Absolute upper-left Y: 0
Relative upper-left X: 0
Relative upper-left Y: 0
Width: 1280
Height: 1024
Depth: 24
Visual Class: TrueColor
Border width: 0

Hope it helps.