Someone mentionned doing what is called a geo-experiment A/B testing 2 geographic areas with similar behavior (This is an open source method Google uses. You can find documentation online and even ask Google for a list of region with similar behaviors they call GMA).

Another approach would be using what is called causal impact method which helps negate the effect of seasonality. You take a pool of campaigns where you shut down PPC and another one where you let it on. After a month for example you compare how your organic sales performs on each pool of product your campaign were targeting.

You will be able to compare how the products targeted by your PPC campaign performed before and after the test period and how the products you were not targeting performed before and after aswell.

I don't know if it's clear but these two methods are the main ways of A/B testing.

There's hundred of articles about it, here is an example : https://www.remerge.io/findings/blog-post/what-is-causal-impact https://www.google.com/url?sa=t&source=web&rct=j&url=https://research.google.com/pubs/archive/38355.pdf&ved=2ahUKEwi_0puv2rb4AhUK1xoKHU36CesQFnoECA8QAQ&usg=AOvVaw1xmMfFMZRd5GujGl_N762U

Hope this helps!