Nouns

(n)tattletale, tattler, taleteller, talebearer, telltale, blabbermouthsomeone who gossips indiscreetly

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”