I'm scheduled to help run a caucus site next Monday night.
The process has us looking up every would-be caucus participant in the county voter database. If they don't show up or, they're not registered Republican, they must register and/or declare as a Republican. I could be mistaken but, I expect proof of residency will be required (as on a normal election day).
Personally, I would be more concerned about the Republican Party of Iowa in Des Moines cooking the books. (It wouldn't be the first time.) But, supposedly, that's been made impossible by utilizing an app to transmit the results from every caucus site and requiring all of the raw data to be retained for later scrutiny.
Question: Is the ballot a secret ballot? I thought I heard that it was, but then I saw old news footage of someone (in a Hillary shirt) counting raised hands for Hillary.