% puts [dispinfo installed]
AccessX Multi-Buffering SHAPE MIT-SHM Shared-Memory Transport
XInputExtension XTEST Keyboard-Management-Extension
MIT-SUNDRY-NONSTANDARD MIT-SCREEN-SAVER SYNC DEC-XTRAP XVideo Xie XIE
Adobe-DPS-Extension DPSExtension

% puts [dispinfo installed AccessX XVideo]
1

% puts [dispinfo installed AccessX No-Such-Extension]
0

Example 3: Android can tell you what extensions are available in your X server, or test to see if some particular extension is present.