Through cookies!
How long are cookies valid for?
The cookies are kept for 14 days. So if any unregistered user clicks on the affiliate link that you post, they will have 14 days to register to become your referral.
What if someone clicks on my affiliate link and then on the other?
The rule of thumb is that if a person clicks on someone else’s referral link within 14 days after clicking on your link, they will still be considered as your affiliate. The only exception to this rule is if, within 14 days after clicking on the link, the person deletes cookies on their computer prior to registration.