Jacob,
A pleasure; if it's the Divisions - Infantry or Armoured - then it's relatively straightforward: the identifying number is a white 40 on a black square (on the left as you look at the front of the vehicle) and on the opposite side the badge of the Division concerned eg the black bull of 11th Armoured, the Wyvern of 43 Wessex Div and so on. These markings will be repeated on the vehicle rear.
I'm no WW2 expert but I think other markings would be a bridge classification plate, a vehicle serial number on the vehicle's doors, presumably a gas detection plate somewhere and I should think that's it.
Once parked up and forming part of the HQ complex, ie functioning in its actual role, a small sign indicating the respective Staff Branch would be displayed, either lodged on the vehicle itself, or, more likely, staked into the ground appropriately (near the access steps). Such signs would be, for example, "G Plans", "G Int" and so on. This is assuming that the HQ was the Main HQ (Formation HQs were divided into 2 components, "Main" and "Rear" - physically separated. The Main HQ contained the Commander and his immediate planning Staff, concerned with the actual warfighting operations.
Amongst the many Staff functions would be "Plans" or "Intelligence" as abbreviated above. The "G" prefix merely reflects the "General Staff" nomenclature.
I suspect such signs would be white on black.
Rear HQ was the component concerned with logistics and Administration etc.
The British Staff system at the time was - by today's relatively simplified NATO standards - horribly convoluted and reflected an historical development almost across the centuries. If you do need further clarification I'll gladly help.
Brian