telltale

telltale    [tel-teyl] 

adjective.  revealing, disclosing important information unintentionally 

"A telltale patch of oil on the water marked where the boat went down."

Scientists have just found a new star, LkCa 15, in formation. It is similar to the sun, but only 2 million years old. This discovery demonstrates a technique to find other baby planets by searching for the telltale hydrogen gas emissions.

"In a telltale quote, the owner said - It’s fair to say quarterback position is open.”