You can't tell by the rib.
My XE grade guns have a beveled muzzle end like this:
I have three Fox XEs (12, 16, and 20) and the barrels don't touch at the muzzle on any of them. And all of the barrels on them match the factory records.
You can get a factory letter from John Callihan in a few weeks for $80 I think. If you are in a big hurry you can join the Fox Collectors and and they will email you a picture of the factory build card. I usually get those within an hour of submitting my request and paying for it with PayPal. You get one free one each year.