The Oxford Thesaurus An A-Z Dictionary of Synonyms - telling adj.

1) effective, effectual, influential, weighty, important, powerful, forceful, potent, significant, considerable, striking: The new colour scheme in the office has had a telling effect on morale. --n. 2) tattling, (too) revealing, Colloq letting the cat out of the bag, giving away the whole show: I cannot say why her key was found in his pocket - that would be telling.