How to recognize AN JIC racing fittings for aero oil and fuel radiator hoses ?

If you are sure it is an AN JIC pitch, you just need to measure the diameter between the thread ridges with a digital caliper:

3AN 9.39mm - Inner bore 3.1mm

4AN 11mm - Internal bore 5.06mm

6AN 14.12mm - Internal bore 8mm

8AN 18.7mm - Internal bore 11.45mm

10AN 21.76 -21.83-21.95mm - Internal hole 12.57-12.65-13.00mm

12AN 26.76mm - Internal bore 15.44mm

How to recognize AN JIC fittings for oil and fuel coolers