How to determine if a phone has liquid damage or not?

With phones with batteries that can be replaced and removed from the battery panel. There will be a white dot on the phone itself, behind the battery. In some instances, the water damage indicator will be on the battery itself, or even on both the phone. If the water damage indicator is WHITE, this means the phone is not water damaged. If the water damage indicator is PINK or RED, this means the water damage indicator has been tripped.

For most Apple phones, the indicator can be found inside the SIM tray slot. Shine a light into the SIM tray after you take out the sim tray by inserting a pin into its pinhole. You can use a flashlight or just hold the phone up to a desk lamp. Look for a red water contact indicator. If the iPhone has contacted a liquid near your SIM tray, you’ll see a red indicator in the center of the tray opening.

On iPhone 7 models, the indicator is a strip that takes about half the slot.
On iPhone 6 models, the indicator is near the center, offset just a little.
On iPhone 5 models, the indicator is round and in the center of the slot.