Yep, arrest/booking reports I'm pretty sure have a box for:
U.S. citizen? Y/N
At least they did when I was typing them. So that is already being done, it's not a new thing for sure. I like the Sherrif of Maricopa county, Arizona. If every jail were run like his, maybe criminals wouldn't be so complacent about doing time.
Sanctuary cities don't detain illegal immigrants. That is kind of the point of a "Sanctuary city". Having a box to check if the person is a U.S. citizen is useless if the cop is not allowed to do anything about it.
I do think it's odd though that sanctuary cities are allowed to openly defy federal law and get away with it but Arizona tries to enforce federal law and it gets shot down.

Any explanations?
On July 31, 2008, 14-year-old Ivan Miranda was murdered and nearly decapitated in a sword attack in San Francisco's Excelsior neighborhood. Authorities arrested Rony Aguilera, known as "Guerrillero," in the attack and believe Aguilera is an illegal immigrant from Honduras.
Aguilera had been arrested in 2007 in an assault case, but was never reported to federal immigration officials due to San Francisco's sanctuary policy.
Sorry Ivan Miranda, you probably didn't have to die, but because liberals in San Francisco think they are so great for not reporting or detaining illegal immigrants, you did.