ok, so its not hypothetical, but its actually not me.  my friend has a phone that was exposed to some rain in a parking lot he dropped it in for awhile (but not in a puddle).    it doesnt have any function problems, but a few weeks after that instance (the phone was obviously dry)  - the pressure switch on the flip phone that senses if it is opened or closed has been sticking.  so he wants a new one.

on the phone they asked if it was exposed to water, and he said no.

now my other friend basically submerged his, and let it dry and it worked.  thats what i consider exposed to water.  they arent made of sugar, and would be uselessif they couldnt be used out in the rain for a little bit.

he has to send the old phone back, and he's wondering if 1. water actually did get in, and 2. if they have a way to tell if water got in.

if the electronics were screwy i can understand that would be obvious, but the problem seems to be purely mechanical.

so, can they tell?  (my guess would be if they had something that would dissolve if wet, so if its not there, they know)

ya. buddy of mine dropped his cell into the pool.  took the whole thing appart and let it dry for a week, then took it back and said that it stopped working.  they took the phone to their shop, and within 24 hours called him and said that the phone had gotten wet and he would be liable to cover the replacement

usually yes. i heard this on a teck pocast just the other day. there is a white dot onder the battery pack on allmost all phones and if it gets wet it turns red.
but i just checked my wifes phone and her lg does not.

They can tell. 2ways

1. on almost all cell phone batteries there is a indicator. either a sticker or mde into the battery. It has a white backgroud with a red pattern across it (look at your battery, you'll see it) when the phone(battery) is exposed to water the red pattern will bleed. thus a indication that it was exposed to water. however I have seen the indicators bleed dew to high humitity.

2 most phones rely on the indicator described in No:1 but some cell phone manufacturers have the same indicator inside the phone itself.

find indicator and remove.

Often electronics that got wet have horrendous corrosion all over the boards as well.  

Circuit boards water stain easily too.