How can you find out if your iPhone ever got wet?
Those sensors are no electrical sensors, but pieces of paper that change from white to red when they get wet. There are up to two spots you can look at without opening the device. Have a look inside your headphone jacket and your dock connector. (use a torch/flashlight as lint or dim light can make these orangish-pink indicators hard to discern!)

Have a look at this page.

Here's the diagram for an iPhone 4/4S: