There is a whole lot in the literature on voting systems, and a good bit of game theory can be applied. The issue that's difficult is that it's inherently probabilistic; you pick certain patterns as indicating probable fraud, and detect or exclude them; by doing so, you also exclude the possibility that someone is voting that way for innocent, or at least non-fraudulent reasons.
Consider, eg, someone who reads my deathless prose, develops an instant man-crush on me, and goes through all my answers voting each one up. I've got more than 30 answers so it would take a few days. Now, by assumption, this isn't my reputation-whoring sock-puppet, it's a person who for their own reasons, however unwise, has devoting all their voting to me for days at a time.
Is this fraud? No, but it would be detected as, and probably treated as, fraud.
