If you haven't already, you should make a referral tracker (or several). Go to My Account > Earnings > Referral Trackers (on the left). HubPages has already given you one, it will look like a bunch of letters and numbers. It is generally advised not to use this one, as it is unattractive and might deter people from actually clicking the link.
Use the Create a New Tracker box to make your own. It should be something that is not suspicious to someone who might click your links, the shorter and more unassuming it is, the better. It might take a while to find one because many are already being used by others.
Once you have created a tracker, you use it by inserting an underscore ( _ ) and the tracker after "hubpages.com/" but before "/hub/name-of-article". For example, if your tracker is "health", you would insert it as:
If anyone clicks on this link, HubPages will know that you were the one who brought them there.