How do you detect fraud aside from checking IPs and cookies?

Question

I got a hunch that someone created another account to vote for himself but I don't have any actual proof. What I've been using so far is checking the mysql database for identical IPs and cookies. In this case, that doesn't help.

What do you do to detect cheaters?

Comments

I am looking for an answer to this too. IP and cookies cannot always be the differentiator as a friend of mine runs a Q2A site which is mostly used by folks from his college and often from public computers in the college where I ended up seeing so many logins from same IPs which is quite obvious.