A person's refusal to do anything that looks like a test can turn into a significant challenge. A childhood onset of a cognitive disability tends to produce bad experiences with tests. Doing an activity that does not look like a test is a big help. Do not say or think the word "test".
Taking a few minutes to talk a new person, when they can talk, is time well invested. A relaxed conversation often gains cooperation and provides better information than standardised interviews and self-reports.
A brief conversation can obtain information about their education, work and family history, while establishing rapport. A classic OT question is to ask the person to describe a recent typical day. Practice different ways of weaving the question into a "casual" conversation.