Furrin’ sounding names will be one way. Comparison with other databases that are supposed to capture only people in the country legally will be another. So they’ll compare against tax records, social security records, commercial address lists etc, and ding the count for anyone who doesn’t appear in those. (Yeah, there are some really obvious practical problems with that even ignoring the concentrated evil.)
This census has been so badly done that I expect in some areas the census data will result in lower counts than captured in some other databases. At which point heads should explode, but won’t.
Robert’s will corral the crew to deny the apportionment gambit and note in his opinion that the federal government must count all peoples. This will ensure states like Texas get their congressional seats. Robert will also note that states may apportion state district’s omitting undocumented immigrants as the poison pill.