Check against existing list?

I have a magazine which is sent to a number of people. Some receive the magazine for free, other have a paid subscription.
The magazine also has a website. We’d like to close part of the website and make it only available to 1) those who have a paid subscription for the magazine and 2) new paying members who will have the choice of also receiving the printed magazine.

Let’s say Faithful Reader is on the website for the first time and after having read a few articles gets the message: ‘Log in or register to continu reading’. He goes to the registration page and fills in the necessary fields. Is there a way that we can check if Faithful Reader already has a paid subscription for the magazine? Because is he does, he immediately gets access to the while site and if he doesn’t he’ll have to pay a subscription.