what's to stop them from lying?
We will consider the speedy reporting of correct precinct vote tallies as proof of attendance.
However, verifying the vote tallies that contestants report could become a non-trivial issue. On Friday I called the WA GOP chairman's office and asked if they would be publishing an Excel spreadsheet with vote tallies by precinct like they did for the 2008 primary, and their answer was essentially "maybe". (In other words, we'll have to see how much fudging of the data is necessary to stop Ron Paul from winning before we decide how transparent we're going to be.) If they do, then determining winners will be pretty straightforward. I'll download the SMS messages from Google Voice, import them into an Excel spreadsheet and compare them to the published results.
Since there is no guarantee that they will publish precinct tallies online, we're going to require contestants to submit the name and phone number of their precinct chairman along with their vote tallies. We can then call up the county chairman to confirm that the name reported is in fact the precinct chairman, and then call the precinct chairman to confirm the tally (or the county chairman might do both). If we get bombarded with bogus reports, then this could become impractical.
If there is no way to verify vote tallies, then we will have to choose winners another way, which we will have to warn is a possibility ahead of time. We could randomly draw phone numbers from those who correctly reported their precinct chairman's name and phone number by some cut-off time (say 11am). This would make it likely that the winner at least attended their caucus. (I suppose people could cheat by calling around to get their precinct chairman's name and phone number ahead of time and then not show up. But anyone who goes through that trouble might as well just go vote for Ron Paul anyway to make it more likely they win the prize, since they have no guarantee that the contest will be decided by random drawing.)
If anyone has any more ideas about how to verify either vote tallies or attendance in case the WA GOP is uncooperative (which is more likely the better Ron Paul does), please share.