Professor Layton and the Curious Village - DS/Quick Strategies

From WikiCheats

Hint Coins

There are a limited number of Hint Coins hidden through out the game. Whenever you reach a new location or "screen", be sure to Tap every nook and cranny you see. Hint Coins are hidden everywhere. Even if you've scoured a location during one chapter and return it later on in another, be sure to go over it again as new Hint Coins will appear in locations. Once you've collected all the Gizmos and completed assembling the robot dog. There will be times when he will appear on the screen and start barking and sniffing around, Tap around that area to find either hint coins or hidden puzzles.

An easy (but also time-consuming) way of getting Hint Coins is to tap every pixel of on the screen to find them.