I’m aware of the coding for the rear differentials. I recently purchased a “J1h” rear diff and I want to verify that this is in fact a 3.307 . What’s a sure fire way to check ? The pinion has 43 teeth

Just check both ring and pinion gear tooth count.
You should have 13T on the pinion and yes you have 3.307.

Ring gear tooth count divided by Pinion gear tooth count = Ratio
43 divided by 13 = 3.307

Thank you sir