Because ive lived in south texas my whole and its plain as day. Everyone here can immediately tell who is local and who is not from around here. Different dialect, different way of dressing, different mannerisms and behavior, etc..
Also the last few years other little details...mexican banks not found here before have been popping up near malls, more signs and stores putting things in spanish, more commercials on tv in spanish featuring latin americans, huge sections of sporting goods stores now selling soccer jerseys where before there were none, latin american pop stars/celebrities showing up in ads, schools being swamped with immigrant kids, whole neighborhoods being bought out by rich mexican nationals..basically we are seeing a gradual transformation into a latin american enclave.
Respectfully, JM all that means is that the ones here aren't leaving (or dying off very quickly).
(And that means that living in Mexico sucks so much that the poor illegals don't realize how much Texas sucks, lol.)
Also, I question how you'd be able to tell if a kid born in the USA to an illegal immigrant younger than about 3rd grade is a local or not if he or she is only speaking Spanish.
I grew up in CA and I've been surprised before in public (like at a laundromat, or on BART) when a grade school kid with his family who's been speaking only Spanish busts out with some perfect but slightly accented English when his mom has him talk to a non-Mexican about something.
Businesses are just trying to make money. If Spanish speaking people have money then it just makes sense for businesses to cater to that market by speaking their language.
From a purely cultural standpoint (not a nationally economic one), I don't really get what the objection is. Yeah, "Texas" culture will change in the border areas, but is that necessarily a bad thing? I love Tex-Mex food, for example. Also, many Mexican females are purty.