Core 0.12 has an RPC call to tell you whether a tx opts in to RBF (which AIUI also checks whether an unconfirmed parent tx opts in).
Armory only sets up an RPC connection to Core when running the auto bitcoind management. I'd prefer a solution that covers ever case.
EDIT: Although perhaps it's only useable if the tx pays to an address in the Core wallet - I haven't looked at what it does so I don't know.
Core keeps track of all ZC tx (otherwise Armory would not be able to see them). I expect the RPC would let you get the RBF flag by tx hash, regardless of the relevance to Core's own wallet.