He is a high priest, and we can not only see divine magic but identify which god it belongs too. If we can do that, he almost certainly can as well, unless Ranald was intentionally hiding himself, which to be fair is something he would do.
But I think it's safe to assume that if he saw the familiar bond, he saw the touch of Ranald as well. It's not like we make any attempt to hide it.
My reading of the "one faithful to another" bit might just be a way of saying "follower [of Ranald] to follower [of Ulric]" instead of "wizard to high priest" or "Dame to an Elector". He was inviting us to speak as an equal.
It's like the Starke thing all over again. The thread assumed that he assumed that we were faithful to Sigmar, when all he did was ask us if we were faithful. And we are faithful. We use our 26pts of Piety to pray to an approved god. Assuming a faithful person in a polytheistic society is faithful to the same god as you is, in my opinion, a little bit foolish, and I can't see either the Ar-Ulric or Starke making that mistake.
It should be noted that even our Volans grade mage sight is not enough to recognize any approved god, only the ones we are familiar with like Ranald, Morr and maybe Mork. Even assuming the Al Ulric had mage-sight as good as ours where would he have gotten significant familiarity with Ranald?