Now you tell me!

cliché A rebuke against someone who failed to share an important, useful, or valuable piece of information earlier. A: "The car started swerving out of control on the highway!" B: "Oh, yeah, there's a problem with the steering." A: "Gee, now you tell me! Thanks a lot—I could have been killed!" A: "You know that there's a way to automate that process in the spreadsheets, right?" B: "So now you tell us. We've wasted the whole morning doing it manually!"