Is there any way I can check what version (for example v1.0.2) of Might and Magic: Heroes 6 is currently installed via the Steam client?
If you want to know the version of the game, you can find it in the main menu in the lower right/left corner.
However, be aware that Steam does not patch Heroes 6.
It's patchable through the Ubisoft Installer application which in turn is installed by Steam when it enters the prerequisite stage of the game installation phase. Whenever you launch HOMM 6 via Steam it automatically launches the Ubisoft application which checks if there is a new patch.