Você pode usar o parâmetro -queryOptions de xdpyinfo :
SYNOPSIS
xdpyinfo [-display displayname] [-queryExtensions] [-ext extension-
name] [-version]
DESCRIPTION
Xdpyinfo is a utility for displaying information about an X server. It
is used to examine the capabilities of a server, the predefined values
for various parameters used in communicating between clients and the
server, and the different types of screens and visuals that are avail‐
able.
By default, numeric information (opcode, base event, base error) about
protocol extensions is not displayed. This information can be obtained
with the -queryExtensions option. Use of this option on servers that
dynamically load extensions will likely cause all possible extensions
to be loaded, which can be slow and can consume significant server
resources.
Ex .:
$ xdpyinfo -display :0 -queryExtensions | awk '/^number of extensions:/,/^default screen number/'
number of extensions: 29
BIG-REQUESTS (opcode: 133)
Composite (opcode: 142)
DAMAGE (opcode: 143, base event: 91, base error: 151)
DOUBLE-BUFFER (opcode: 145, base error: 152)
DPMS (opcode: 147)
DRI2 (opcode: 154, base event: 102)
DRI3 (opcode: 149)
GLX (opcode: 155, base event: 104, base error: 169)
Generic Event Extension (opcode: 128)
MIT-SCREEN-SAVER (opcode: 144, base event: 92)
MIT-SHM (opcode: 130, base event: 65, base error: 128)
Present (opcode: 148)
RANDR (opcode: 140, base event: 89, base error: 147)
RECORD (opcode: 146, base error: 153)
RENDER (opcode: 139, base error: 142)
SECURITY (opcode: 137, base event: 86, base error: 138)
SGI-GLX (opcode: 155, base event: 104, base error: 169)
SHAPE (opcode: 129, base event: 64)
SYNC (opcode: 134, base event: 83, base error: 134)
X-Resource (opcode: 150)
XC-MISC (opcode: 136)
XFIXES (opcode: 138, base event: 87, base error: 140)
XFree86-DGA (opcode: 153, base event: 95, base error: 164)
XFree86-VidModeExtension (opcode: 152, base error: 157)
XINERAMA (opcode: 141)
XInputExtension (opcode: 131, base event: 66, base error: 129)
XKEYBOARD (opcode: 135, base event: 85, base error: 137)
XTEST (opcode: 132)
XVideo (opcode: 151, base event: 93, base error: 154)
default screen number: 0