The first Thief was released multiple times and not all of my tutorials work for all versions of Thief. I'm currently running Thief 1.37 (the first release of Thief Gold) and most of my articles reflect that.
Open the folder where you installed Thief.
Find THIEF.EXE and Right-Click it with your mouse.
On the pop-up context menu, click on Properties near (or at) the bottom.
Click on the Version tab.
Select Product Version in the left-hand list under "Other version information".
Voila! In the text box to the right of the list (under "Value:") the version will be displayed.