If my forebears were from Mexico, Honduras, Peru, I would have one way of imagining how the new Arizona immigration law might affect me. How could a policeman be sure, on sight, that I hadn't just sneaked across the Sonoran desert from Mexico? Why shouldn't he ask for my papers, just to find out?
Although my forebears are instead from Scotland, England, Germany, I can still imagine a little of what it would be like. I just have to think back to being in China.
The situations are different in one obvious way. In contrast to law-enforcement officers in Arizona, the Chinese authorities didn't have to waste time wondering whether I was a citizen. One glance told them where I stood. (I understand that there are some Caucasian-looking Chinese citizens, but they are scarce.) The only judgment call was whether they should bother to check whether, well, my "papers were in order," in the phrase we all know from WW II movies.
If they had checked very often, I would have been in trouble. In theory, foreigners are always supposed to carry their passports (as Chinese citizens are supposed to carry their identity cards). In practice, I almost never did. When checking in for a flight or registering at a hotel in China, sure: Without a passport, you couldn't do either thing. But when at "home" in Shanghai or Beijing my wife and I kept our passports in our apartment's safe. The theoretical risk of being asked for documents was outweighed by the truly dire potential consequences of our passports getting lost or stolen.