The use of key codes is the best place to start. Here is the link: https://www.sfimg.com/Support/Solution?id=19365
Another way to track your ads could be the use of a survey. You can ask your affiliates how they found out about SFI so you can find out which method of advertising worked. Other than that, I am not sure of any other way so I will leave that up to others to add suggestions with their answers.
