telltale
1. [ adjective ] disclosing unintentionally
Synonyms: telling revealing
Examples:

"a telling smile" "a telltale panel of lights" "a telltale patch of oil on the water marked where the boat went down"

Related terms: informative
2. [ noun ] someone who gossips indiscreetly
Synonyms: taleteller talebearer blabbermouth tattler tattletale
Related terms: gossip tattle