You have no way of knowing that.
(a.) You can always ask them, and they are usually pretty straightforward in telling you because they have pride in where they came from. [and] (b.) You can tell from their accent and their slang speaking Spanish, if you have an ear for it.
In case you're not aware, there's lots of ignorant comments on this thread, including some racist comments.
You have no way of knowing ANYthing that aint posted. And he posted that they were MEXICANs who can barely speak english. We DO have a way of knowing the probabilities of them being here on H2B status: 84,000 out of about 3 million this year, which is less than 1/30 chance of them bein’ legal and all that.
Once we get to the point that it’s a 1/30 chance that they’re ILLegal then we can reopen the borders to those indentured servants...