Perhaps from polls such as exit polls.
Not really.
You have a better chance of finding them ‘at home’ than your average harried, errand-running citizen. 
First of all, most Americans aren’t registered with any party at all. About half the states don’t even have party registration; in them you just choose which party primary to vote in on election day. You just ask for that party ballot, or often they are printed on different sides of the same ballot.
So it’s quite possible to vote in the other party’s primary. But that assumes that:
- there isn’t any contest in your own partys primary (and often, primary battles are because of open seats, and so both parties have a primary contest).
- you know for sure which candidate in the other party will be the weakest opponent for your party. That can be hard to determine – to a republican, is Hillary Clinton or Barack Obama the weaker democrat? This can be a risky choice.
Canadian prison inmates can vote (unlike the US, Canada’s Constitution includes a “right to vote” clause) and are basically considered to be residents of wherever they were living before they were jailed.
>How, I ask, does anyone know who Republicans in Texas voted for?
I think they said this information came from exit polling, as Giles guesses.
But Republicans voting crossover might not want to identify as such in polls, for various reasons. Conceivably Obama supporters, anticipating a loss, would identify themselves to pollsters as Republicans so as to give the impression that Clinton’s win was tainted.
>Conceivably Obama supporters, anticipating a loss, would identify themselves to pollsters as Republicans so as to give the impression that Clinton’s win was tainted.
Well, yes, that’s plausible. There are actually quite a few different possible strategies for choosing parties, voting, and answering pollsters. It is almost painful to try to think them all through. A more realistic goal might be for parties to recognize this as a new kind of “error”, collecting primary votes the real meaning of which is actually that the voter holds the candidate they vote for in the lowest possible regard rather than the highest. I would expect a remedy to this would be requiring voters to already be registered in the party before voting in its primaries (as many states already do). But, I don’t know. Sometimes I find it much more comfortable to do my homework, place my vote, send my donations, and then stick my fingers in my ears and say “ICan’tHearYouICan’tHearYou” fast for a few minutes to forget it all.
Both of these are possible, true. Hard to imagine that even one percent of those interviewed would bother, though. You’re being lumped into an anonymous poll. Nobody knows what you answer. Why would you believe that so many others are going to lie - in the correct way, and not canceling each other out - that they would have any effect on the final numbers?
So. Possible, but very, very doubtful. It’s more something that gets talked about paranoically online than happens in real life.
Here in Ohio, while Obama did get some crossover Republicans to vote for him, there were also large numbers that voted for Clinton. The Cleveland Plain Dealer did some research and interviewed some of the people that did the switching. Some voted for Obama because they thought McCain has a better chance of beating him, voted for Clinton to help keep her in the race and weaken the Democratic Party, and some voted for the 2 candidates because they thought they were the best person to vote for.
If you were wondering how the people that did this were found, they needed to sign a form saying they were switching their party allegiance to the Democratic party to vote in the Democratic primary. And technically, all those people that crossed over and voted for a Democratic candidate while intending to vote for McCain in the fall were breaking the law in Ohio. It has happened before, and no one is ever called on it, but the law is still there.
There was a recent newspaper column by a conservative writer in the local paper who suggested that Republican voters change their registration to Democrat and vote for Clinton. His thinking was that this would help weaken the Democrats because it would prolong the campaign.
However, he failed to remember (or maybe he didn’t care) that the primary in this state is also the primary for state and local elections. So any Republicans following his advice will be unable to vote for their candidate for state offices.