Step 1 · seat
What is on the end?
Gender
Thread form
Male thread OD
Caliper reading
Threads per inch
Result
Measure to identify
Put a caliper on the first full thread. Male: outside. Female: inside. Skip TPI if you have to — OD plus the seat is usually enough.
1/2″ BSP and 1/2″ NPT both read about 21 mm and both are 14 TPI. Check taper and the 55° / 60° flank before you force it.