There are several possibilities:

(a)

In SAP GUI window, click on
-> Customize Local Layout
-> About ... 
-> Loaded DLLs

Scroll until you see guixt.dll. The version is shown in the column "Product version".

(b)

In GuXT window click on
-> Help
-> About

If the GuiXT window is not visible you may use the command

guixt.exe  visible

to make it visible.

In the SAP GUI folder

C:\Program Files\SAP\FrontEnd\SAPgui

or for 64bit Windows

C:\Program Files (x86)\SAP\FrontEnd\SAPgui

scroll down until you see guixt.exe or guixt.dll. Then click

-> Right mouse button
-> Properties
-> Details