telltale

telltale (http://definr.com/telltale)

     adj : disclosing unintentionally; "a telling smile"; "a telltale
           panel of lights"; "a telltale patch of oil on the water
           marked where the boat went down" [syn: revealing, telling,
            telltale(a)]
     n : someone who gossips indiscreetly [syn: tattletale, tattler,
          taleteller, talebearer, blabbermouth]