Telltale signs are indicators or cues that reveal something is happening or about to happen. There are several synonyms for the term "telltale sign" which include "clue," "hint," "signal," "indication," "mark," "evidence," "symptom," "manifestation," "proof," and "trace." These synonyms can be used interchangeably depending on the context of the sentence. However, each synonym has a slightly different connotation and meaning. "Proof" and "evidence" imply a more concrete basis, while "clue" and "hint" suggest a more subtle indication. "Symptom" and "manifestation" refer to physical or observable changes, while "signal" and "indication" refer to something that communicates a message.