Whether you sell a phone to us or are purchasing another phone, it is good to know how to check for water damage. Most if not all phones have a water damage indicator. For many phones it is behind the battery or within the slot where the battery stays. You can google to find what color should show at a certain location to show water damage.
For iPhones, there are multiple water damage indicators. If an iPhone has been fully submerged, within the headset dock you will see the indicator turn pink or red. There is also an indicator within the charging port at the bottom of the iPhone. These indicators are available in all models including iPhone 3G, 3GS, 4, 4S, and iPhone 5. The iPhone 5 also has an indicator within the nano SIM card slot. The best way to look at these indicators is with a small flashlight. If there is another smartphone in the house you can always download a flashlight app and look at your phone that way. (Some of our technicians will do that from time to time). You could also purchase one of those cool tiny flashlight keychains and use that to check your phone.
If your phone does get wet, do not try charging it or connecting it to iTunes or something to restart it. Let it be. Make sure your hands are clean and dry and then use a cloth to dry off your phone. Clean the phone through and through. What I mean by that is make sure that the headphone dock is as dry as you can make it. Wipe off your SIM card and inside the slot as well. If you are trying to remove your SIM card and do not have a SIM pin you could alway use a needle or a push pin or something with a point that you could place in the tiny hole to eject the SIM card. Lastly, submerge the phone within a bag or bowl of uncooked rice and let it sit for about 48 hours. After this time period has passed, you can then take your phone out of the rice, insert your SIM and try using it again. Depending on how severe the water damage is, your phone may not be fully functional.
If you are curious as to how this rice thing works, there are many youtube videos about this.
Good luck, and happy selling!