Unlike it’s predecessors the iPhone 6 and iPhone 6 Plus only have 1 internal water damage indicator/sensor. This sticker is on the logic board, at the top in the middle; right below the front display/screen.
On the iPhone 6 Plus it is horizontal:
On the iPhone 6 it is vertical:
In the pictures it is white, if you had water damage “officially” it would be red/pink.
Your Daily Overdose 
false, there is also a water damage indicator under the sim card slot. remove the sim card slot when the iPhone is opened and you’ll see it
Pablo is correct. There is one in the sim card slot and the other is on the motherboard as the author states. I’ve not been able to observe one in the charge port or the headphone jack after looking at a VERY water soaked iPhone 6.
I havent seen it out there but there is another one that i saw on my iphone 6s. When you remove the home button assembly and flip it, there should be a very small one to the right of it. You’ll be able to tell. Keep in mind, it’s very small.