Drop in a prototype link and watch somebody try to finish the flow, days before a single component gets built.
Point it at the live signup and see where new accounts stall, usually on the step everybody agreed was obvious.
Two labels, one question, three hundred people. Comprehension results come back the same afternoon, and the argument about the word ends with a number.
Show three directions before any of them is designed, and find out which one people can describe back to you unprompted.