The Collaborative International Dictionary of English v.0.48:
Telling \Tell"ing\, a. Operating with great effect; effective; as, a telling speech. -- Tell"ing*ly, adv. [1913 Webster]WordNet (r) 3.0 (2006):
tellingly adv 1: in a telling manner; "the plain manner of its style all the more tellingly points up the horror of the case"