Numerous organizations (right, left, and otherwise) have asked their members to vote absentee and look to have been successful. So trying to separate the wheat from the chaff based strictly on the number of absentee ballots is not going to help much.
But I will tell you what could work. Having the parties or other concerned organizations clean the registration lists. Voter registrations are usually public records, and the parties have the right to challenge voter credentials. The party or other organization can send a bland campaign postcard to each new registered voter. You send it DO NOT FORWARD, FORWARDING ADDRESS REQUESTED. Each postcard you get back is suspect registration. It's not fool proof, but it does work. And it costs about 25 cents each.
Keep in mind that you can vet darn near anyone on the net these days if you have their name and address. If it's a ghost voter, you won't find other records that they exist. Again, a suspect registration.
The question in my mind on this stuff is WHERE ARE THE PROSECUTIONS? Political organizations should use their clout to demand accountability and enforcement of the law. I haven't heard much in this regard. Send a few people to prison and word WILL get around.
The correct defense is for that 527 discussed yesterday to offer substantial rewards to anyone who fingers a fraudulent voter. Those vans going around and the absentees would be at considerable risk of being turned in for a felony.
The kind of outrage that would result in prosecutions would likely not happen unless there was a national groundswelling of enormous proportions. I myself, regardless of the outcome, will start a letter writing campaign and visits to local offices if necessary to try and force our elected officials to deal with the issue.
Challenging voter registrations is a tricky process.
I worked as an election official in Vermont for 10 years, and the voter lists could only be 'purged' during certain periods. I know that 'Motor Voter' changed this, but I believe it is still pretty well defined as a process and not as easy as it sounds.