The simplest quick way to confirm that a Series 2/2a is ex army is the raised suspension that was fitted to them. To do this, they had a second hole for the bolt at the front of each spring about 50mm above the standard hole (the standard height one is there as well), and the spring shackles are extended, with a bolt and spacer through them halfway between the two shackle bolts.

In addition, the flywheel housing crossmember has a cutaway to clear the front prop shaft, and the rear crossmember is different; it does not have the step down bit in the middle of the bottom that the civilian ones do. (But rear crossmembers are prone to rust, and may have been replaced with the wrong one)

There are quite a few other differences, including cutaway front mudguards, for example, but as they are all on bolt on bits, they are not definitive - it is hard to hide or fake the chassis differences, and I have not seen any where it has been tried.

There are quite a few ex-army 2as that look civilian at a first glance (including mine) and I know of several civilian ones dolled up to look ex-army. Some of these are just a coat of paint, others have gone to a lot of trouble, but the chassis will almost always give them away.