Well you can’t ask one’s marital status on a job interview. I guess the interviewee would have to be wearing he/she clothing and a rainbow flag pin. Usually seven different colors of hair give it away as well.
I remember a quick interview I did with a guy of Asian decent at one of those college job-fair days in Florida. My company did a lot of classified work so I asked him if he was eligible to work in the United States. He was outraged! He yelled, "I'm an American citizen"! He calmed down after I told him I wasn't allowed to ask that.