>What test would there be for deciding between "ible" and "able"...

It is difficult to say when we are not to use -able instead of -ible. "Yet a rule may be laid down as to when we are to use it. To all verbs, then, from the Anglo-Saxon, to all based on the uncorrupted infinitival stems of Latin verbs of the first conjugation, and to all substantives, whencesoever sprung, we annex -able only.'' --Fitzed. Hall. - Webster's Revised Unabridged Dictionary, © 1996, 1998 MICRA, Inc.

HTH.