Although this is not an exact measure but I check my traffic. If I am sending them a good amount of leads and nothing is converting, I know they are stealing my sales.
This happened to me once with a speech/toast ebook. My website was specifically about wedding toasts and I made good conversions on the page with a couple products. But interestingly enough, no matter how much qualified traffic I sent this guy, I never had any sales. I quickly removed his affiliate link. | 
Denied Offers from Affiliate Sites 
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