Replacing a screen with water damage

I recently dropped my LG G3 into some water. It worked fine with no incident for about 2 weeks, until I started getting black flickering lines across the screen. This was also joined with the screen fading to darkness and then the screen turning off altogether. This has worsened now and it is rare to get the screen working for more than 30 secs a day.

As it responds to my touch still, I assume it is the lcd screen that is broken and not the digitiser (although I'll replace both if I repair it). My problem is, I don't want to spend £80ish on a new screen, to find out it is actually water damage to the motherboard which would make the repair to expensive for me. Any suggestions to determine whether it is screen damage or motherboard? And whether it is worth the rish?

Thanks