Nouns

(n)relation, telling, recountingan act of narration“he was the hero according to his own relation”, “his endless recounting of the incident eventually became unbearable”
(n)telling, apprisal, notificationinforming by words
(n)tattle, singing, tellingdisclosing information or giving evidence about another

Adjectives

(s)revealing, telling, telltaledisclosing unintentionally something concealed“a telling smile”, “a telltale panel of lights”, “a telltale patch of oil on the water marked where the boat went down”
(s)cogent, telling, weightypowerfully persuasive“a cogent argument”, “a telling presentation”, “a weighty argument”
(s)impressive, tellingproducing a strong effect“gave an impressive performance as Othello”, “a telling gesture”