How to distinguish between unauthenticated evaluators of an article

I am providing a possibility to evaluate a web article to anonymous readers. I need to prevent the same user performing the evaluation several times and want to inform him/her that they already evaluated the article. How should I distinguish between these unauthenticated users. If I go for IP I am afraid I will disable evaluation for all the users accessing with the same IP (using proxy and NAT). Could you recommend another ways?