Cash back sites.

Does anyone know how cash back sites/apps confirm that someone had signed up. Are they setting up some kind of call back with the vendor?

Examples I am thinking are about are sites like Quidco or apps like StormX.

I started to implement something similar with my high payout affiliate links but I have no idea if the user actually signs up after clicking my link.

Any thoughts or guidance is appreciated

You've to use tracking links, some vendors also ask for cookies