Pretty sure all G29s are running iDrive 7; the easiest way to tell visually (apart from the overall 'look') is to check the home screen. If the tiles are different sizes and there's a column of icons down the left-hand side (for media/comms/nav etc), that's iDrive 7. iDrive 5/6 had no icons on the left, just a horizontal row of tiles which were all the same size.
The very latest models such as the iX, i4 and new 2AT are running iDrive 8 instead; the big giveaways there seem to be that the instrument cluster and centre screen look like they're combined into one surface, and the aircon controls have been moved onto the touchscreen instead of having dedicated buttons. A big change like that probably wouldn't happen for the G29 except maybe as part of the LCI, but even that seems pretty unlikely for a small-volume car like the Z4.