Referral tracking

Hi,

I am doing a project using now. Its not online yet. I am doing a registration form which involve online payment for a membership website. We have a few partners that will refer this registration form page in their website.

The scenario now is one partner is asking for commission for any member referred from their website and we don’t have problem with that. My problem now is how can i track the customers that is came from their website?

I can do a drop down field asking the customer to choose how they know about us and then list down the option like Google, Newspaper, Partner A website, Partner B website, etc. The problem with this method is i think the customers will not bother about the options. They may just choose Google even if they came from Partner A website.

How can i solve this problem? Usually website with affiliate program do this kind of referral tracking. How can i do this to my website?