A "telltale signal" refers to a clear and noticeable indication or sign that reveals something important or provides useful information. Full definition
Popular stereotypes and generalities have played a role, priming parents and teachers to look for telltale signals of autism such as hand flapping and verbal tics, failure to make eye contact, or a laser - like fixation on trucks or dinosaurs. (edutopia.org)
Others can identify aircraft, missiles and satellites by searching for telltale signals in the electromagnetic spectrum. (newscientist.com)
But the team calculates that if the layer of rock and icy soil above the water table is particularly conductive, it could be absorbing enough energy from the radar to obscure a telltale signal. (newscientist.com)