For that you usually hire a market research company or do what they will do: take an interviewer, two cameras (one front-face, one top-hands) and hire an as-diverse-as-possible pool of test candidates that you then put through whatever workflow optimization you want to do. Then afterwards, you interview them - side benefit, you can get really interesting general side knowledge that you'd never gain from a dumbass A/B scheme: is your font style/color scheme legible, can the site be used by colorblind people, are there stock photo choices that give off stereotypical vibes...
It's real fun and a worthwhile experience for everyone involved.