Special prices when shopping thru affiliate – need to check for affiliate

[Cope]

I have the problem that customers shopping thru an affiliate link - eg. http://nyshop.dk?mitaffiliateid must have a different price. 

 

The affiliate part works, but I need to check if there is an affiliate currently active. 

I guess it is stored in a cookie, but I need to check if there IS one, so that I can give the discount. 

 

I cannot use any of the affiliate modules I have found so far, as they tie a customer to a particular affiliate (in my case an agent) - customers should be free to switch and buy from another agent that then gets the percentage. 

 

I have figures I can make the special prices thru use of a special, "secret" currency, which can only be used when one has entered the shop via an affiliate and therefore I need to know if we are currently shopping with an affiliate active. 

 

All help is welcome. Is there a freelancer out there with knowledge of the affiliate system and the price system, and needs  projects, I have more!