Households that do not return the census forms are personally visited by Census Takers (I was one in the 2000 Census). I was often presented with Mexican Photo IDs when filling out the forms here in Central Florida. I had far more problems with drunks and druggies cooperating than with illegals!!!
The forms are confidential by law (as if that matters in 2019...) and do not become public for 72 years after filed.