If the OP is not a naturalized citizen, the answer is simple: Green card or other proof of legal status must be carried at all times.

If the OP is a naturalized citizen, then I share the same concern: Running into some La Migra thug who will detain anyone not fitting into his preconceived idea of how US Citizens are supposed to look and talk.

And if anyone doubts, a quick Google search for "American citizen deported" or "US citizen deported" should be convincing enough of what these goons are capable of.

Here is a comment from
http://www.detentionwatchnetwork.org/node/842, which I think sums it all up:

"We have heard of dozens of us citizens who have been wrongfully arrested, detained or deported. Why is this happening? Because the Department of Homeland Security is bent on removing people perceived as 'un-American. But what can be more un-American than the outright denial of due process and fundamental fairness?"

I think La Migra is even more un-American than TSA.