Clearly there can be no "probable cause" to stop someone for "looking Mexican"...that is a major Equal Protections and Due Process problem...
Remember also...any "stop" is technically an arrest... And an arrest needs probable cause...
Clearly...just "looking Mexican" will not be enough...there needs to be something else in the mix...
Likely this law will be used by police ...at night...in and around Hispanic neighborhoods...to check out suspicious acting people..which of course could be anyone...
Likely anyone who could legally be asked for their drivers license...can be asked for citizenship proof... And if the law is written so it applies to everyone...say Australians or Germans...then it has a better chance...
Curiously ...the law would be more shaky if it were passed in Nebraska which does not have a big issue with illegals...