How does wp-polls work (What is PHP's Anonymous Personalization methods and best practices)

I know the title might be a bit broad but I chose it to encourage further coverage on this topic since there's not enough resources covering it.

What I've been told is that there's only 3 ways to recognize each user :

• Cookies : whether it's the usual cookie or a cookie containing session ID

• -URl : containing the usual GET data or session ID IP Adress

• User's IP Address

But I tried to workaround wp-polls(which is wordpress plugin for polls/surveys) just to see if I could vote more than one time (only for research purposes I swear :D)

this is the URL(I'm sorry it's not English but I'm sure you'll find the plugin it's the on the left below the nav-bar)

so I tried

• Deleting the cookies that was sent by the website containing that plugin
• check the URL (didn't see any session IDs)
• Browsing the website containing the plugin using a proxy server

The plugin detected me after deleting the cookies,( but when I browsed using the proxy server it detected me on that page but when I went to the Homepage and clicked the vote button it didn't do anything[rather than saying "you've already voted"] probably the proxy server conflicted with the Javascript of the ajax call)

My question is .. how does that plugin work and detect users (did it stopped working on purpose when I used the proxy server?) and what are the methods/best practices to detect users ?

via Chebli Mohamed