If you want to tell if your iPhone 3g has liquid damage, the first 2 spots you would look would be:
1) The Earphone/headphone Jack = Put a flashlight or shine a light in there, directly in the center is a sticker which should be white. If it is red, then it means it has touched some sort of liquid there.
2) The Charger Port = Flash a light or flashlight aorund the chargerport, underneath it there is also a sticker toward the inside where theres a little opening, that sticker should be white as well. If it is red, this indicates it has some sort of liquid damage.
These are the 2 ways to tell if an iPhone 3g has Liquid/Water damage without opening the phone up. Keep in mind, moisture can cause these stickers to change color as well, so you wont know 100% unless you open up the phone and look for any rust on the main board or any components. There are also two stickers, one on top half and one on lower half of the main board which indicate water damage (white = good, red = water damage). Again keep in mind, just because these might be pink or red, it could just be because someone always left their phone in the bathroom while taking a shower and the heat and moisture caused these to go red or pink, doesnt mean the phone is bad or doesnt work, you would have to test the phone out to ensure it has actually been dipped into water. Hope this helps :)