Finding compatible fixed-gear cog and lockring
You are right that there is no magic way to tell which standard a hub is made to, if the manufacturer does not tell. If the threads are not damaged, calipers are indeed the best way to find out which standard the hub is. The thread pitch difference between English and ISO/Italian is too small to detect with pitch gauge.

The difference is small enough and aluminum soft enough that you can brute force a smaller thread cog on a larger hub. If someone already did this, measurement will get you the smaller size. It would be better to stick to the smaller standard afterwards.

With lock ring things are easier for two reasons: First, the standards are so different that measuring is easy. Second, you can just use the lock ring that came with the bike unless you managed to destroy it.