Various methods, mainly based on your IP address (the unique 'address' of your computer or internet connection). I also look at who the users vote on, if they're voting normally on lots of different people then it's unlikely they are the same person and cheating (ie to vote their own answers).
I've found several members with seemingly multiple accounts, haven't really decided what to do about it yet. It could be different people, if many people are using the same internet connection.