How to find out which app is providing adverts (without reverting to process of elimination) I'm on Android 6 (P8 if it matters) and I am getting very annoying adverts.

Screen shots show it all:

 
Screenshots (click images for larger variants)

This icon shown on the right of the first screenshot with the word WIN persists on my "desktop" or "home" screen too. If I click on it, I get what's shown in the second screenshot – as pops up enough to be annoying but it doesn't pop up daily. 

My question is, how do I know what is causing this so I can remove it.

The only option I know of is to start removing apps and use the power of elimination but that is not ideal! 

The other option I know of is to click on it - sometimes an advert  is part of the app installed (in this image, it takes me to the buy plug ins from the Keyboard app I use) which probably suggests that this app is causing it (but not guaranteed). 

Regardless of those 2, I'm still keen to know how to tell, be it via logs or anything else.
I'm pretty sure that your keyboard is cause of ads showing (I guess this isn't stock one, is it?) try finding settings of that keyboard and see if there is option to turn off ads (I guess there isn't but still worth of try) however if you don't find option to turn off ads than simply uninstall some reliable keyboard (G BOARD and such)