How can I tell if a restaurant or business hires illegal Mexican immigrants?

As a big opponent to illegal immigration in california I face many difficult choices every day. It is because of concerned people like me that the problem will alleviate. Illegal immigration is a immense problem in California and causes quite a backlash these days, not only from generally ‘Anglos Americans’ and other groups to latinos, but also simply from legal Mexicans who are (regrettably) looked at with increased negativity.
Many times I go to a restaurant or other business like store or carwash and I see a lot of latino (usually Mexican) workers, waiters, valet etc. I won’t mind if they are legal but I fear they are mostly not these days. How can I tell? I need to know because spending money in the wrong places and people stimulates illegal immigration.
Must I ask (and believe) the owner? Do illegals have certain tattoos (3 dots)? I saw a post saying that if you shout ICE, those who scatter are illegals. Seriously i don’t want to shout ICE in every restaurant. Any better way?

Actually you can not tell if a person is legal or not. There’s no sign on their forehead saying I’m illegal.