Take out the SIM card and put it and the phone in a bag of rice for 48 hours. Don't try and turn it on and don't try and plug it in or charge it. No guarantee this will work, but it has worked on other iPhones and cell phones.
Supposedly there are two visible moisture sensors, arranged in and under the transparent bottom of the jack as one half-circle facing out, one half-circle on the inside. This is so you can see how far the moisture got.