To verify the thread is BSPT, you would undergo the same steps as for a parallel British thread. First, verify the number of threads per inch. Second, measure the O.D. at the 3-4th thread from the end of the fitting. You can cross-reference your measurements to the following chart in...
parallel Metric threads is that an O.D measurement will reveal the actual thread size (i.e. 18.01mm measurement means the thread is likely M18). However, to do this accurately with Tapered Metric threads one must measure the O.D at the third row of threads from the end of the fitting....