From what I understand, in Indiana you're allowed to challenge a particular voter if you believe they are voting illegally. I found this little tidbit while browsing around:
In any Primary election in INDIANA, voters state which Party they are affiliated with by asking the poll workers for the ballot of that Party in a given Primary. A voter's party affiliation is determined solely by his or her Primary voting record; the state's voter registration form does NOT provide a place for the voter to state Party affiliation.
Even though the history of a voter's Party affiliation- solely as determined by the individual voter's Primary voting record- is part of the public record, the informaton is NOT available at the polling place. Any voter, however, may challenge another voter's Party affiliation at the polls:
In such a case, the voter making the challenge (the "challenger") fills out an affidavit that the voter being challenged (the "challengee") has not, in fact, voted in the Party's Primaries in the past, an assertion against which the challengee must fill out a counter-affidavit, asserting that they intend to vote for a majority of the candidates of the challengee's Party of choice in the next General Election. If the challengee fills out this counter-affidavit, the challengee is allowed to vote in the Primary; however, if the challengee refuses, he or she is not allowed to vote in the Primary.
Nevertheless, since General Election ballots are cast in secret, the intention expressed in the counter-affidavit by the challengee is impossible to enforce.http://www.thegreenpapers.com/P08/IN-D.phtml