How to tell if an iphone has been damaged by water?

Help!
I bought a second hand iphone 4 yesterday. The seller told me that the phone was okay and he sold it to me for £170. And now I noticed that there's a yellow thing on the left side of the screen. I don't know if you can see it on the photo, you can zoom it in if you want. Do you think this phone has been damaged by water or is there something wrong with LCD?

If something's wrong with it, i don't think it shud worth £170. It should be cheaper than that -___-
How to tell if an iphone has been damaged by water - 1

There's an explanation on the link to show where to look to find out about water damage
https://www.gazelle.com/how-to/iphone/water-damage

I suspect the problem with your phone is not water damage.

Wow, you got ripped off. £170 for a used iPhone 4? Such iSheep.

Its not water damaged your display is fake, i've experienced, sign of water damage is your iphones is take torch and see inside charging dock and headphone jack if its red or pink inside its definitely water damaged
but its display problem