So, if I run ads or a marketing campaign, I shouldn't be able to know if it brings a positive ROI or not? Let's assume I pay $1000 for Google Search Ads, would

So, if I run ads or a marketing campaign, I shouldn't be able to know if it brings a positive ROI or not? Let's assume I pay $1000 for Google Search Ads, wouldn't it help the business to know that "from my sales, $800 came from Google Search Ads"? People do this all the time, even in real life, with coupon codes fliers for example.