Thread identifier
Three questions, working from geometry rather than dimensions — taper, thread angle and seat. Answer them with a caliper and a pitch gauge.
Step 1
Measure across the thread crests at the first thread and at the last. Does the diameter change?
This single measurement separates the two tapered families from everything else.
This narrows a fitting to a family, not a part number. Size and series still need measuring, and on a port you cannot easily re-machine it is worth confirming before you commit the joint.
For the full method, including what each family seals on and the pairing that leaks, read how to identify any hydraulic fitting in four steps.