Unless your iPhone has pentalobe screws, you should be able to pop the back off by removing the two phillips screws to the left and right of the dock connector. For the pentalobes, you can try a really tiny flat blade or just grabe the pentalobe tool from iFixit.
P2 Pentalobe Screwdriver iPhone
As long as you do it properly, Apple will not know that you've been in your phone. Would it technically violate the warranty, yes... but the genius bar would never be strict about that unless you actually broke something.
The best place to check the liquid sensors are 1 - in the headphones jack, 2&3 - along the left side of the battery. Of course, you'd have to pop the backplate off to see those.
Personally, I wouldn't have any reservation about opening it to let it dry out. Check those liquid sensors and go from there. Good luck!