One simple approach is leaderboards, similar to Top Contributors, 30 Days, that's something like Top Upvoters, 30 Days and Top Downvoters, 30 Days. This would give them a sense of who's driving karma shifts (and would identify potential problems, like VoiceOfRa massdownvoting people, as they're happening).

It seems likely these should only be available to the police (i.e. the mods)--you don't want people voting just to make their score higher.

A more direct approach deals with the vote graph directly. Serial upvote and downvote detection, as done by Stack Overflow, relies on the graph, but sockpuppets are probably more noticeable by something like voting cliques or vote distributions. It would be interesting to take other people's vote relationships with me (i.e. one person may have upvoted 100 of my posts and comments and downvoted 10 of them) and figure out what sort of distribution that takes on, and then see if there are users with odd distributions (of their votes on others or others votes on them). A well-known similar approach is Benford's Law; if one has received a disproportionate number of votes from a small number of users, or given a disproportionate number to a small number of users, then it is likely that some sort of sockpuppetry is going on.