By learning what's what, although there are part number charts that can be used to help with thread identification.
Some but not all, thread forms were also changed from British Standard to Unified during Commando production.
Here is one 'identify by part number' list (these lists do contain the odd error)
http://stainlessbits.com/link12a.html
BA = British Association. A 47.5 degree thread often used for fasteners under 1/4".
https://britishfasteners.com/threads/index.html
https://en.wikipedia.org/wiki/British_Association_screw_threads
This is what I mean by knowing your threads as a Commando can have a mixture of UNF, UNC, UNEF, UN, CEI/BSC, BSW, BSF, BA, ME and BSP threads depending on the actual component and time of production, plus spark plugs and spin-on filter threads are metric (also speedo and tacho cable
upper end fittings and 850 Mk3 switch cluster screws).
(Edit: Possibly also UNS but would be for lock ring threads, access plugs, etc.)