When you search for a game, it will display the current price in addition to the regular price. If it's on sale, it will also show you what percentage you're saving (for example if it's 50% off or 25% off). It also displays the lowest price that it's been listed at so you can compare that to the current prices offered.
Instead of just searching Steam, Is There Any Deal searches multiple sites including GamersGate, IndieGala Store, GamesPlanet, FunStock Digital, Amazon, and Humble Store in addition to Steam. This lets you compare prices between the sites to make sure you get the game you're looking for at the best price.