A telltale sign is an indication of something, often a warning or a sign of trouble. An AROMA is a pleasant smell, a SCENT is a distinctive ODOR, a tipoff is a clue or hint, an EARMARK is a distinguishing feature, an OMEN is an indication of future events, an ODOR is a smell, and a HALLMARK is a distinguishing feature. All of these words can be used to describe a telltale sign, which is an indication of something that may be coming.