I have come across a few iPhones that have had liquid damage with both indicators have been set off but yet the phone still works fine. Opening your iPhone consists of taking the back cover off first. Apple has not placed any counter measures to ensure people do not take the back cover off so they won't know any different. They have however placed a liquid indicator smack bang on the side in the middle of the logic board on top of a screw which is needs to be unscrewed in order to to take the logic board out. There is a way to get around this by using a nail file to carefully peel it off but before you get into that you need to visually look at a few things.
1. Is there rust? If so where?
If there is rust on the screws it generally means that a small area around that has been effected as well. If there is a lot of rust then you will have very little chance of getting it working again.
2. How pink is the liquid indicator? If it is almost red then things aren't looking good. If it is only slightly pink then your logic board will have suffered minimal liquid damage and it could just be as easy as replacing the digitiser (screen) because it has been damaged.
Basically there is no DEFINITE way to know until a qualified iPhone repairman has had a look and has run diagnostics. If it was me this is what i would do. Keep in mind i do have every iPhone part at my disposal.
1. Does the iPhone turn on?
2. Connect to itunes, is it recognised?
3. Look at the Liquid indicators/signs of rust.
4. Replace the battery, does the iPhone turn on?
If any of the above have worked i will then start testing different areas of the phone.
5. look for damaged connections (LCD/Digitiser/vibrator etc.) if none then continue to step 6.
6. Disconnect the Digitiser/LCD and test with a new one
If none of the steps above have worked then i class the iPhone as unrepairable as i do not do logic board swaps.
By having liquid damage on your iPhone you have already voided the warranty so apple won't care if you do mess with it, but if there is a chance of getting it repaired legitimately while keeping your warranty then that would be my preference!
Hope this helps :)