Other forms: tellings
Something that's telling is either significant,or it reveals information. Getting twice as much allowance will have a telling effect on your ability to buy new books.
When it comes to economics, a telling indicator of a country's state is how much money its citizens spend each month. And you can see that a movie has had a telling effect on its audience if it leaves them all weeping. If you reveal something without meaning to, that's telling as well: "The look on his face was telling." This adjective dates from the mid-1800s, from the verb tell and its sense of "to reveal or disclose."