You ask them the “illegal” question in the line of other LE work you are doing. If something comes to your attention as you deal with them on other LE matters. An exception might be if you see a bunch of day laborers gathered where past experience says illegals are hired, then that LE matter coincides with the “illegal” question. Otherwise, you do NOT look at someone and say to yourself, hey, an Hispanic, gotta ask them if they’re legal.
If more states start to pass/adopt this kind of a bill as well as start the process or rejecting Health care the WH will have a full blown Revolution on its hands....