Você pode usar xdpyinfo :
screen #0:
dimensions: 1366x768 pixels (361x203 millimeters)
resolution: 96x96 dots per inch
depths (7): 24, 1, 4, 8, 15, 16, 32
root window id: 0x2b9
depth of root window: 24 planes
number of colormaps: minimum 1, maximum 1
default colormap: 0x20
default number of colormap cells: 256
preallocated pixels: black 0, white 16777215
options: backing-store NO, save-unders NO
largest cursor: 64x64
current input event mask: 0xda4033
KeyPressMask KeyReleaseMask EnterWindowMask
LeaveWindowMask KeymapStateMask StructureNotifyMask
SubstructureNotifyMask SubstructureRedirectMask PropertyChangeMask
ColormapChangeMask
number of visuals: 240
default visual id: 0x21
visual:
visual id: 0x21
class: TrueColor
depth: 24 planes
available colormap entries: 256 per subfield
red, green, blue masks: 0xff0000, 0xff00, 0xff
significant bits in color specification: 8 bits